CBSE Class 9 Chemistry Atoms And Molecules Worksheet

Read and download free pdf of CBSE Class 9 Chemistry Atoms And Molecules Worksheet. Students and teachers of Class 9 Science can get free printable Worksheets for Class 9 Science Chapter 3 Atoms and Molecules in PDF format prepared as per the latest syllabus and examination pattern in your schools. Class 9 students should practice questions and answers given here for Science in Class 9 which will help them to improve your knowledge of all important chapters and its topics. Students should also download free pdf of Class 9 Science Worksheets prepared by school teachers as per the latest NCERT, CBSE, KVS books and syllabus issued this academic year and solve important problems with solutions on daily basis to get more score in school exams and tests

Worksheet for Class 9 Science Chapter 3 Atoms and Molecules

Class 9 Science students should refer to the following printable worksheet in Pdf for Chapter 3 Atoms and Molecules in Class 9. This test paper with questions and answers for Class 9 will be very useful for exams and help you to score good marks

Class 9 Science Worksheet for Chapter 3 Atoms and Molecules

Exam Questions NCERT Class 9 Science Chapter 3 Atoms And Molecules

Question. What is meant by the term chemical formula?
Ans : The chemical formula of a compound is a symbolic representation of its composition and actual number of atoms in one molecule of a pure substance may be an atom or a compound

Question. How was the relative atomic mass determined?
Ans : Relative atomic masses were determined by using the laws of chemical combinations and the compound formed.

Question. What is the number of electrons in Mg atom and Mg2+ ion?
Ans : Mg = 12e–
        Mg2+ = 10e–

Question. Define valency.
Ans : The combining power of an element to attain the noble gas configuration is called valency. Or, it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.

Question. Name the instrument which produces image of the surface of element that shows atoms.
Ans : Scanning tunnelling microscope.

Question. The relative atomic mass of oxygen atom is 16. Explain its meaning.
Ans : The relative atomic mass of an atom is the average masses of the atom, as compared to th 12 1 the mass of one carbon-12 atom.

Question. Why atoms form ions?
Ans : Atoms get stability by acquiring the stable electronic configuration of the nearest noble gas for which either they lose electrons or gain electrons and thus acquire noble gas configuration.

Question. Write the postulate given by the Indian philosopher Maharishi Kanad.
Ans : Indian philosopher Maharishi Kanad postulated if we divide matter we will get smaller and smaller particles. He said that a time will come when we come across smallest particles beyond which further division will not be possible.

Question. Who was the first scientist to give the concept of formation of compounds?
Ans : Antoine L. Lavoisier gave the concept of formation of compounds.

Question. Define the term gram atom. How is it related to mole and Avogadro number?
Ans : The atomic mass of an element expressed in grams is called gram atomic mass. One gram atom of any element contains 6.022 × 1023 atoms of the element. It is equal to one mole of atoms. One gram atomic mass = 6.022 × 1023 atoms = 1 mole

Question. Name the gas which gives the lightest positively charged particle.
Ans : Hydrogen.

Question. 50 g of 10% lead nitrate is mixed with 50 g of 10% sodium chloride in a closed vessel. It was found after reaction that 6.83 g of lead chloride was precipitated. Besides, the reaction mixture contained 90 g water and sodium nitrate. Calculate the amount of sodium nitrate formed.
Ans : 50 g of 10% lead nitrate = 5 g lead nitrate
                                                  + 45 g water
50 g of 10% sodium chloride = 5 g sodium chloride
                                                  + 45 g water
Total content before reaction = 5 + 5 + 90 = 100
  Total content after reaction = 90 g
          Amount of precipitate = 6.83 g
According to law of conservation,
Total mass of reaction mixture = 100 g
Amount of sodium nitrate = 100 – 90 – 6.83 = 3.17 g

Question. What are ionic compounds?
Ans : Ionic compounds are charged particles. Such compounds form by joining or losing or sharing the electron.
For example : Sodium chloride is an ionic compound. Its constituent particles are positively charged sodium ion (Na+) and negatively charged chloride ion (Cl–).

52. What did Antoine L. Lavoisier observe regarding the formation of compound?
Ans : He noted that many compounds were composed of two or more elements. Each compound had the same elements in the same proportions.


Question. Define one mole, give its relationship with Avogadro constant.
Ans : One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 1023. This number is called Avogadro constant or Avogadro number.

Question. Who stated the Law of Constant Proportion?
Ans : Joseph Louis Proust stated the Law of Constant Proportion.

Question. Name any two monatomic atoms.
Ans : Sodium, Aluminium.

Question. Write the value of charge of electron.
Ans : 1.6 × 10–19 coulomb.

Question. Name the element which is used as the reference for atomic mass.
Ans : Carbon. 

Question. What are polyatomic ions? Give two examples.
Ans : A group of atoms having a charge is known as polyatomic ion.
        Examples : (NH4)+ (SO4)2–

Question. Explain the law of multiple proportions.
Ans : According to law of multiple proportions, when two elements combine to make one or more compounds then the ratio of weights of these element remain in fixed ratio to one another. For example : Hydrogen and oxygen combine to form water (H2O) and hydrogen peroxide (H2O2) under different condition. 2 grams of hydrogen combines with 16 grams of oxygen in case of water while 2 grams of hydrogen combines with 32 grams of oxygen to form hydrogen peroxide. Now, the weights of oxygen combine with a fixed weight of hydrogen in water and hydrogen peroxide respectively are 16 and 32 which are in simple ratio of 16: 32 or 1 : 2.

Question. What is meant by a molecule? Give examples.
Ans : A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance, e.g., molecule of oxygen is O2, ozone is O3, phosphorus is P4, sulphur is S8, etc.

Question. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Ans : 1 g of hydrogen reacts with oxygen = 8 g 3 g of hydrogen reacts with oxygen = 8 × 3 g= 24 g

Question. Define formula unit mass. Calculate formula unit mass of NaCl (atomic mass of Na = 23u, Cl = 35.5u).
Ans : The formula unit mass is same as molecular mass which is equal to the sum of masses of atoms present in a formula unit. Formula unit mass of NaCl = (23 + 35.5) = 58.5u.

Question. Give two examples of triatomic molecules.
Ans : Carbon dioxide (CO2) and water (H2O)

Q1 What are polyatomic ions? Give examples.

Q2 Write the chemical formulae of the following compounds.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride

Q3 Calculate the molar mass of the following substances
(a) Hydrogen molecule, H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4
(d) Nitric acid, HNO3
(e) Hydrochloric acid, HCl

Q4 What is the mass of
(a) 1 mole of nitrogen atom
(b) 4 moles of Aluminium atom
(c) 10 moles of sulphuric acid

Q5 Convert into moles
(a) 12g of oxygen gas
(b) 20g of water
(c) 22g of carbon dioxide

Q6 Write the cations and anions in the following compounds
(a) CH3COONa
(b) NaCl
(c) HNO3
(d) MgCl2

Q7 Verify by calculating that
(a) 5 moles of carbon dioxide and 5 moles of water do not have the same mass.
(b) 240g of calcium and 240g of magnesium elements have a mole ratio3:5 .

Q8 Compute the number of ions present in 5.85g of sodium chloride.

Q9 What are ionic and molecular compounds? Give examples.

Q10 What do you understand by the term atomicity?

Q11 Write the formulae for the following and calculate the molecular mass for each one of them
(a) caustic potash
(b) Baking powder
(c) limestone
(d) Ethanol
(e) Common salt

Q12 Give the chemical formulae of the following compounds and compute the ratio by mass of the combining elements in each of them
(a) Ammonia
(b) Carbon monoxide
(c) Hydrogen chloride
(d) Aluminium fluoride
(e) Magnesium sulphide

Q13 Does the solubility of the substance cha nges with temperature? Explain with the help of an example.

Q14 Give the formulae of the compounds formed from the following sets of elements
(a) Calcium and fluorine
(b) Hydrogen and sulphur
(c) Nitrogen and hydrogen
(d) Carbon and chlorine
(e) Sodium and oxygen
(f) Carbon and Oxygen

Q15 Wrire all the postulates of Dalton’s atomic theory.

 Exam Questions NCERT Class 9 Science Chapter 3 Atoms And Molecules

Question. How does an atom exist?
Ans : Atom exists in the form of atom, molecule or ions.

Question. Define law of constant proportion.
Ans : Law of constant proportion states that, ‘In a pure chemical substance, the elements are always present in definite proportions by mass.’

Question. State the number of hydrogen atoms in 1 g of hydrogen.
Ans : One gram of hydrogen = One mole = 6.022 × 1023 atoms

Question. State the law of conservation of mass.
Ans : This law states that the mass can neither be created nor destroyed in a chemical reaction. That is ‘bass of reactants is always equal to mass of products.

Question. Who introduced the word ‘Mole’?
Ans : ‘Wilhelm Ostwald’ introduced the word ‘Mole’.

Question. What is Avogadro Constant?
Ans : The number of particles present in one mole of any substance is fixed with a value of 6.022 × 1023.

Question. What is the full form of IUPAC?
Ans : International Union of Pure and Applied Chemistry.

Question. Write atomicity of the following : (i) Sulphur, (ii) Phosphorus
Ans : (i) Polyatomic,
         (ii) Tetra atomic.

Question. What is the difference between an atom and molecule?
Ans : An atom is the smallest particle of an element which may or may not have independent existence. For example : Na, Al, Fe, etc. Molecule is the smallest particle of the element or compound which can exist independently. For example : O2, H2, N2, etc. 25. Name two elements which have same atomic number. Ans : Two elements cannot have the same atomic number.

Question. What is molar mass? What are its units?
Ans : The mass of one mole of a substance is called its molar mass. Its unit is gram per mole (gmol–1).

Question. Give an example in each of the following cases :
(i) a divalent anion
(ii) a trivalent cation
(iii) a mono-valent anion.

Ans : (i) O2– (ii) Fe3+ (iii) I– 

Question. What is the use of mole concept?
Ans : Applications of mole concept :
(i) We can calculate the number of basic particles from the number of moles as the number of moles of a substance is directly proportional to the number of elementary particles.
(ii) One mole of gas occupies 22.4 litres at 273K.
(iii) One mole of any gas occupies the same volume at same pressure and temperature.
(iv) One mole is equal to 6.022 × 1023 atoms. So, we can calculate the absolute masses of atoms and molecules. 

Question. Give the postulates of Dalton’s atomic theory.Ans : Every element is composed of extremely small particles called atoms. Atoms of a given element are identical, both in mass and properties. Different chemical elements have different kinds of atoms; in particular, their atoms have different masses. Atoms cannot be created, destroyed or transformed into atoms of other elements. Compounds are formed when atoms of different elements combine with each 115. 

Question. Give one relevant reason, why scientists choose th 16 1 of the mass of an atom of naturally occurring oxygen as the atomic mass unit?
Ans : Initially, 1/16th of the mass of naturally occurring oxygen was taken as the atomic mass unit because this unit gave masses of most of the elements as whole numbers.

Question. Explain the form of atoms in a solid.
Ans : A solid element is a cluster of atoms. The property of solid does not depend on a single atom but on cluster of atoms. For example : Diamond and graphite though both are composed of carbon atoms but due to different arrangements of carbon atoms in these. They are different in physical and chemical properties.

Question. Give two drawbacks of Dalton’s atomic theory.
Ans : Drawbacks of Dalton’s atomic theory :
(i) According to modern theory, atom is not the ultimate indivisible particle of matter. Atoms are divisible, i.e., they are themselves made-up of particles (protons, electrons, neutrons, etc.).
(ii) The assumption that the atoms of the same element have same mass does not hold good, in case of isotopes of an element.


Question. Calculate the mass of the following :
(i) 2 moles of carbon dioxide.
(ii) 6.022 × 1023 molecules of carbon dioxide. 

Ans : (i) Molar mass of CO2 = 12 + 2 × 16 g = 44 g
1 mole of carbon dioxide = 44 g
2 mole of carbon dioxide = 44 g × 2 = 88 g
(ii) Molar mass of CO2 = 44 g
         = 6.022×1023 molecules of carbon dioxide

Question. State two examples in each case and write their chemical formulae :
(a) Molecules having same kind of atoms only.
(b) Molecules having two different kinds of atoms.
(c) Molecules having three different kinds of atoms.
Ans : (a) F2, Cl2, P4, S8
         (b) Ammonia (NH3), Sulphur dioxide (SO2), Carbon disulphide (CS2).
         (c) Calcium sulphate (CaSO4), Sodium nitrate (NaNO3).

Question. Find the number of atoms in the 0.5 mole of C atom.
Ans : 0.5 mole of C atom :
Number of atoms in 1 mole of C atom
                              = 6.022 × 1023 atoms
Number of atoms in 0.5 mole of C atom
                             = 6.022 × 1023 × 0.5
                             = 3.011 × 1023 atoms

Question. What are molecules? Give brief explanation of the arrangement of the constituent atoms in the molecules.
Ans : A molecule is the smallest particle of an element or compound which is stable in normal conditions. And it can freely show all the properties of that element or compound. It may be made up of one, two or more atoms. Molecule with one atom called monoatomic. E.g. helium, neon, etc. Molecule with two atoms called diatomic. E.g.
Cl2, O2. Similarly, there are molecules containing three atoms (CO2), four atoms (P4) and so on.

Question. What is the ratio by mass of combining elements in H2O, CO2 and NH3 ?
Ans : H2O ratio by mass of combining elements
                                           = 2 : 16 = 1 : 8 (H : O)
CO2 ratio by mass of combining elements
                                           = 12 : 32 = 3 : 8 (C : O)
NH3 ratio by mass of combining elements
                                           = 14 : 3 = 14 : 3 (N : H)

Question. Write the name of the compounds : NaBr, Al2O3, CaCO3.
Ans : NaBr = Sodium bromide
        Al2O3 = Aluminium oxide
       CaCO3 = Calcium carbonate

Question. Why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas? Explain.
Ans : The number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas because hydrogen molecule is diatomic, i.e., a molecule of hydrogen consists of two atoms of hydrogen, whereas helium is monatomic.

Question. Name the gas which gives the lightest positively charged particle.
Ans : Hydrogen.

Question. What do we get if 3 atoms of oxygen unite into a molecule, instead of usual 2 ?
Ans : We get (O3) ozone.

Question. Define atomicity.
Ans : The number of atoms present in one molecule of an element or a compound is known as its atomicity.

Question. Give difference between 2H and H2.
Ans : 2H indicates 2 atoms of hydrogen and H2 indicates one molecule of hydrogen.

1.State ‘Law of constant proportions’.

2.Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

3.Write the molecular formula of a diatomic gas and a triatomic gas.

4.Name the term used for the symbolic representation of a molecule of an element or a compound?

Give an example of it.

5.In the reaction between copper sulphate and sodium sulphide solutions, when 15.9 g copper sulphate completely reacts with 7.8 g of sodium sulphide it is observed that 9.5 g of copper sulphide is formed. What is the mass of sodium sulphate solution formed?

6.Why, the symbol of gold is Au?

7.Calculate the formula unit mass of NaHCO3 (At.mass of Na=23u,H=1u,C=12u,O=16u)

8.Define Avogadro’s constant. Give its value.

9.What is the ratio between masses of carbon and oxygen in CO2?

10.Define atomicity.

11.Nitrogen and hydrogen are present in ammonia in the ratio 14:3 by mass , what would be the mass of nitrogen if the actual mass of hydrogen is 9g. ( CBSE 2014)

2 Mark Questions:

1.Silver nitrate solution was mixed with 5g of sodium chloride solutions to verify law of conservation of mass.8.1 gram of silver chloride was formed and sodium nitrate formed was equal to half of the amount of silver nitrate solution used. What is the amount of AgNO3 used and NaNO3 formed.

2. (i) What is the significance of symbols?
(ii) What do the abbreviations 2O and O2 stands for?

3. (i) Taking the example of water, explain the law of constant proportions.
(ii) Which postulate of Daltons’ atomic theory explains this law?

4. What are polyatomic ions? Give any two examples.

5. Give the names of elements present in:-
(i) Phosphine (ii) Hydrogen bromide (iii) Quick lime (iv) Baking powder.

6. Write the formulae and names of compounds formed by combination of:
i) Fe3+ and SO42- ii) NH4+ and CO3 2-

7.Using the valencies, write down the chemical formulae of the following compounds:
(i) Calcium nitrate (ii) Lead acetate (iii) Silica (iv) Baking soda

8. Define following :- (i) Relative formula unit mass (ii)atomic mass unit

9. Define mole and find the number of moles present in 12 g of oxygen gas.

10. (i) How many grams of chlorine are contained in one mole of chlorine?
(ii) How many molecules are there in 1 g of chlorine? (Gram atomic mass of Chlorine = 35.5 g)

3 Mark Questions:

1. What are ionic and molecular compounds? Give examples.

2. Find the ratio by mass of the combining elements in the following compounds.
i ) MgCl2 ii) NH 3

3.Calculate the mass of 0.5 mole of sugar (C12H22O11) ( Atomic mass of, C=12u, H=1u, O=16u).

4.Name the Ionic compound that is obtained from sea water and Compute the number of moles present in 5.85 g of this compound.

5.What is the atomicity of sulphur. Calculate the number of moles in 34 g of sulphur molecule.(At.mass of S= 16 u )

6. Calculate the number of moles in (i) 3.011 x 1023 atoms of C .

(ii) 9g of water [ NA = 6.022 x 1023 mol-1, At. Mass of O = 16 u, H=1u and C = 12 u]

7. Which has more number of particles 48g of Mg or 7g of N. (At mass of Mg = 24.0u, N = 14 u)

8. State the postulates of Dalton’s atomic theory.

9. Calculate the number of atoms in 120g of calcium and 120g of iron. Which one has more number of atoms? ( At.mass of Ca = 40 u , Fe= 56 u)

10. a) Calculate the number of molecules in 50g of CaCO3..
b) Calculate the mass of 0.5 moles of nitrogen gas.

Value Based Questions:

1. We should take milk every day because it contains protiens and calcium. Some children do not take milk.
Their bones are weak and their growth is also slow. Then will have joint problems in old age. If you don't like milk, you can take curd,paneer and other milk products.
a) Name the compound present in our bones which contains calcium.
b) How will you convince those children to take milk, who do not take milk?
c) Which vitamin is needed for absorption of calcium by our body?

2. Some people take lot of common salt in their food. They use junk food like pizza, burger, cheese and butter which contains lot of common salt. Pickle, sauce, papad also contain lot of common salt. You must be knowing lot of people are having high blood pressure these days and even young people are suffering from heart ailments.
a) What is the main compound present in common salt?
b) Why should we take less common salt?
c) How will you convince your parents to take less common salt?
d) Give one use of common salt.

Case I : Read the passage given below and answer the following questions from 37 to 39.

The knowledge of valencies of various radicals helps us to write the formulae of chemical compounds. The total positive charge on positive ions (cations) is equal to the total negative charge on negative ions (anions) in a molecule. Therefore, in writing the formula of a compound, the positive and negative ions are adjusted in such a way that the total number of positive charges of positive ions (cations) becomes equal to the total number of negative charges of negative ions (anions).

There is another simple method for writing the formulae of ionic compounds. In this method, the valencies (or positive or negative charges) of the ions can be ‘crossed over’ to give subscripts. The purpose of crossing over of charges is to find the number of ions required to equalise the number of positive and negative charges.

Question. Element X has two valencies 5 and 3 and Y has valency 2. The elements X and Y are most likely to be respectively
(a) copper and sulphur
(b) sulphur and iron
(c) phosphorus and fluorine
(d) nitrogen and iron.
Answer : D

Question. The formula of the sulphate of an element X is X2(SO4)3. The formula of nitride of element X will be
(a) X2N
(b) XN2
(c) XN
(d) X2N3
Answer : C

Question. The formula of a compound is X3Y. The valencies of elements X and Y will be respectively
(a) 1 and 3
(b) 3 and 1
(c) 2 and 3
(d) 3 and 2
Answer : A 

Case II : Read the passage given below and answer the following questions from 40 to 43.
A mole of an atom is a collection of atoms whose total mass is the number of grams equal to the atomic mass. Since equal number of moles of different elements contain an equal number of atoms it becomes convenient to express the amounts of the elements in terms of moles. A mole represents a definite number of particles viz, atoms, molecules, ions or electrons. This definite number is called Avogadro number or Avogadro constant which is equal to 6.022 × 10 23 . Hence a mole represents 6.022 × 10 23 particles of the substance. One mole of substance represents one gram-formula of the substance. One mole of a gas at standard temperature and pressure occupies 22.4 litres.

Question. How many grams of sodium must be taken to get 1 mole of the element ?
(a) 23 g
(b) 35.5 g
(c) 63.5 g
(d) 46 g
Answer : A

Question. What is the mass in grams of a single atom of chlorine? (Atomic mass of chlorine = 35.5)
(a) 6.54 × 1023 g
(b) 5.9 × 10–23 g
(c) 0.0025 g
(d) 35.5 g
Answer : B

Question. How many number of moles are there in 5.75 g of sodium ?
(Atomic mass of sodium = 23)
(a) 0.25
(b) 0.5
(c) 1
(d) 2.5
Answer : A

Question. What is the mass in grams of 2.42 mol of zinc? (Atomic mass of Zn = 65.41)
(a) 200 g
(b) 25 g
(c) 85 g
(d) 158 g
Answer : F

Case III : Read the passage given below and answer the following questions from 44 to 46.

The molecular mass of a substance is the relative mass of its molecule as compared with the mass of a carbon–12 atom taken as 12 units. The molecular mass of a substance indicates the number of times one molecule of a substance is heavier than 1/12 of C-12 atom. It is equal to the sum of atomic masses of all the atoms present in a molecule. Depending on the number of atoms of same or different elements present in the molecule, it can be monoatomic, diatomic, triatomic, tetratomic or a polyatomic molecule.

Question. Which is an example of a polyatomic molecule?
(a) S8
(b) HNO3
(c) C2H5OH
(d) All of these
Answer : D

Question. Total number of atoms in 44 g of CO2 is
(a) 6.02 × 1023
(b) 6.02 × 1024
(c) 1.806 × 1024
(d) 18.06 × 1022
Answer : C

Question. Carbon dioxide, hydrogen sulphide, calcium chloride and sodium oxide are examples of
(a) triatomic molecules
(b) triatomic and tetratomic molecules
(c) diatomic and triatomic molecules
(d) tetratomic molecules.
Answer : A

Case IV : Read the passage given below and answer the following questions from 47 to 50.

According to Dalton’s atomic theory, all matter whether an element, a compound or a mixture is composed of small particles called atoms which can neither be created nor destroyed during a chemical reaction. Dalton’s theory provides a simple explanation for the laws of chemical combination. He used his theory to explain law of conservation of masses, law of constant proportions and law of multiple proportions, based on various postulates of the theory. Dalton was the first scientist to use the symbols for the elements in a very specific sense. When he used a symbol for an element he also meant a definite quantity of that element, that is one atom of that element.

Question. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
(a) Atoms can neither be created nor destroyed.
(b) Each element is composed of extremely small particles called atoms.
(c) All the atoms of a given element are identical.
(d) During chemical combination, atoms of different elements combine in simple ratios.
Answer : A

Question. Which postulate of Dalton’s atomic theory explains law of definite proportions?
(a) Atoms of an element do not change during a chemical reaction.
(b) An element consists of atoms having fixed mass and the number and kind of atoms in a given compound is fixed.
(c) Different elements have different kind of atoms.
(d) Atoms are of various kinds.
Answer : B

Question.  “If 100 g of calcium carbonate (whether in the form of marble or chalk) is decomposed,56 g of calcium oxide and 44 g of carbon dioxide are formed.” Which law of chemical combination is illustrated by this statement?
(a) Law of constant proportions
(b) Law of conservation of mass
(c) Law of multiple proportions
(d) Law of conservation of energy
Answer : B

Question. When 5 g calcium is burnt in 2 g oxygen,7 g of calcium oxide is produced. When 5 g of calcium is burnt in 20 g of oxygen, then also 7 g of calcium oxide is produced. Which law of chemical combination is being followed?
(a) Law of conservation of mass
(b) Law of multiple proportions
(c) Law of constant proportions
(d) No law is being followed.
Answer : C

 Exam Questions NCERT Class 9 Science Chapter 3 Atoms And Molecules

Question. Give Latin name of Silver.
Answer : Latin name of Silver is ‘Argentum’.

Question. If 9 g of water is decomposed, how many grams of hydrogen and oxygen are obtained?
Answer : If 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

Question. Name the two laws of chemical combination.
Answer : Law of conservation of mass and law of constant proportions.

Question. Why is it not possible to see an atom with naked eyes?
Answer : Because an atom is too small, i.e., the atomic radii of an atom is of the order 10–19 m to 10–9 m.

Question. Define the atomic mass unit.
Answer : The mass of th 12 1 part of C-12 is equivalent to one atomic mass unit. Previous, it was denoted by symbol ‘amu’ but nowadays it is denoted by symbol ‘u’.

Question. Which organisation approves the names of elements all over the world?
Answer : International Union of Pure and Applied Chemistry (IUPAC).

Question. What is Avogadro number?
Answer : The number 6.022 × 1023 is referred to as Avogadro number and is denoted by symbol NA.

Question. What is the similarity between chlorine molecule, nitrogen molecule and hydrogen molecule?
Answer : Chlorine molecule, nitrogen molecule and hydrogen molecule are diatomic molecules. These are formed by the union of two atoms of the same element.

Question. What is the symbol of the element of molybdenum?
Answer : ‘Mo’ is the symbol of the element of molybdenum.

Question. What is the latest short form of atomic mass unit?
Answer : The latest short form of atomic mass unit is u, according to IUPAC.

Question. Write the chemical symbols of two elements which are formed from the first letter of the elements’ name.
Answer : N (Nitrogen),   F (Fluorine),   I (Iodine),    O (Oxygen) (any two).

Question. Write the meaning of these formulae : (i) 2O, (ii) O2, (iii) O3
Answer : (i) 2O = Two atoms of oxygen
(ii) O2 = One molecule of oxygen
(iii) O3 = One molecule of ozone

Question. Why does the atomic mass of an element not represent the actual mass of its atom?
Answer : Atomic mass of an element is the mass of its atom on the atomic scale, on the other hand, the actual mass of an atom is obtained by dividing the atomic mass by Avogadro’s number.

Question. 8.4 g of sodium bicarbonate on reaction with 20 g of acetic acid liberated 4.4 g of carbon dioxide gas into the atmosphere. What is the mass of the residue left?
Answer : Total mass of reactants = 8.4 g + 20 g = 28.4 g
Total mass of reactants = Total mass of products
28.4 = 4.4 + x (mass of residue left)
28.4 – 4.4 = x ⇒ x = 24 g

Question. How many ions are there in 80 g of magnesium oxide?
Answer : Number of moles of magnesium oxide
= Mass of magnesium oxide / Molar mass of magnesium oxide = 80/40 =2
MgO Mg2+ + O2–
From the equation, we can see that 1 mol of MgO contains 1 mol of Mg2+ ions and 1 mol of O2– ions. 2.0 mol of MgO will contain 2.0 mol of Mg2+ ions and 2.0 mol of O2– ions.
Hence, 2.0 mol of MgO contains 4.0 mol of ions.Number of ions = 4 × 6.022 × 1023 = 2.4088 × 1024

Question. How many particles are represented by 0.25 mole of an element?
Answer : 1 mole of an element contains 6.022 × 1023 particles 0.25 mole of an element contains = 6.022 × 1023 × 0.25
= 1.5 × 1023 particles

Question. What are the postulates of Dalton’s atomic theory. Give at least four points.
Answer : The postulates of Dalton’s atomic theory may be stated as follows :
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of simple whole number to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.

Question. What is meant by a chemical formula? Give examples.
Ans : A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element, e.g., chemical formula of ammonia is NH3, water is H2O and carbon dioxide is CO2.

Question. The atomic mass of an element is in fraction.” What does it mean?
Ans : If the atomic mass of an element is in fraction, this means that it exists in the form of isotopes. The atomic mass of such element is the average of atomic masses of its isotopes and is generally in fraction.

Question. When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. What mass of magnesium oxide will be formed when 3.00 g magnesium is burnt in 5.00 g of oxygen? Which law of chemical combination will govern your answer? State the law.
Ans : When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. It means magnesium and oxygen are combined in the ratio of 3 : 2 to form magnesium oxide.
Thus, when 3.00 g of magnesium is burnt in 5.00 g of oxygen, 5.00 g of magnesium oxide will be formed and the remaining oxygen will be left unused. It is governed by law of definite proportions.
It states that in a chemical substance, the elements are always present in definite proportions by mass.

Question. Calculate the following in 5.6 g of nitrogen :
(a) Number of moles of nitrogen
(b) Number of molecules of nitrogen
(c) Number of atoms of nitrogen
Ans : (a) Molar mass of nitrogen = 14 g
                     5.6 g of nitrogen = . 14 = 5 6 = 0.4 mole
         (b) 28 g of nitrogen = 6.022 × 1023 molecules
              5.6 g of nitrogen = 2.15 × 1022 molecules
         (c) 14 g of nitrogen = 6.023 × 1023 atoms
              5.6 g of nitrogen = 4.30 × 1023 atoms

Question. Write the chemical symbols of two elements which are formed from the first letter of the elements’ name.
Ans : N (Nitrogen),   F (Fluorine),   I (Iodine),    O (Oxygen) (any two).

Question. What is the atomicity of argon?
Ans : Mono atomic.

Question.. Distinguish between molecular mass and molar mass.
Ans : The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule, whereas the mass of 1 mole of any substance is called its molar mass.


Question. Give the symbol of copper, silver, gold, oxygen, zinc.
Ans : Copper –
         Cu Silver – Ag
         Gold – Au
         Oxygen – O
         Zinc – Zn

Question. What is the building block of all matter?
Ans : Atom is the building block of all matter.

Question. What is the measuring unit of atomic radius?
Ans : Nanometre (nm) is the measuring unit of atomic radius.

Question. What is the difference between hydrogen chloride and nitrogen molecule formation?
Ans : Hydrogen chloride is molecular compound and formed by the union of different kinds of atoms while nitrogen is diatomic molecule and formed by union of two atoms of same kinds.

Question. Name two atoms which exist as independent atoms.
Ans : Noble gases such as argon (Ar) and helium (He) exist as independent atoms.

 

1M

1. The atomicity of noble gases is
(A) Mono-atomic
(B) Octa-atomic
(C) Tri-atomic
(D) Tetra-atomic

2. Write the formula of one polyatomic cation alongwith its valency. .

3. What is the mass of carbon dioxide released in the reaction ?

4. Calculate the number of molecules of carbon dioxide present in 4-4 g of it.

5. Law of Constant Proportions was given by
(A) Newton
(B) Rutherford
(C) Lavoisier
(D) Proust

6. 5g of Calcium (Ca) contains [Atomic mass of Ca = 40u]
(A) 1/8mol
(B) 1/7 mol
(C) 1/8g/mol
(D) 1/9mol

7. 1 mole of NH3 molecules contains
(A) 6.022X1023NH3 molecules
(B) 6.022X1022 NH3 atoms
(C) 6.022X1023 NH3 atoms
(D) 6.022X1022 NH3 molecules

8. Unit of molar mass is
(A) Gm/Molecule
(B) Gm
(C) Moles
(D) gm/mol

9. Valencies of iron are
(A) 2 and 3
(B) 1 and 3
(C) 1, 2 and 3
(D) 1 and 2

10. Cuprous sulphide is the chemical name of
(A) CuS
(B) Cu2S
(C) CuS2
(D) Cu3S

11. Mass of 0.5 M of oxygen atom is:
(A) 16 g
(B) 32 g
(C) 4 g
(D) 8 g

12. An ionic compound is made up of
(A) Metal-nonmetal
(B) Cation-Cation
(C) Anion-Anion
(D) None of the above

13. Write the chemical formula of "Aluminium phosphate".

14. Atom consists of electron, proton and neutron, which means it is divisible. This statement
(A) Strengthens Dalton's Atomic Theory
(B) Weakens Dalton's Atomic Theory
(C) Has no effect on Dalton's Atomic Theory
(D) None of the above

15. When 100g of CaCO3 was heated, it decomposes to yield 56g of CaO and 44g of CO2 . This statement proves that
(A) Mass is conserved during a chemical reaction
(B) There is complete loss of CaCO3 during a chemical reaction
(C) Mass of reactant is different from mass of product
(D) CaCO3 decomposes into Ca and CO2

16. Write down the formulae of aluminium sulphate and ethanol.

17. Which are called the building blocks of matter?

18. Write the chemical symbols of -
(c) Tin
(d) Mercury

19. How many metres are there in 1 nanometer (nm) ?

20. Define atomicity.

21. Atomicities of phosphorus and ozone are respectively:
(A) 4, 3
(B) 2, 2
(C) 2, 3
(D) 4, 2

22. What do we call those particles which have-
(a) Less electrons than the normal atoms.
(b) More electrons than the normal atoms.

23. Cation is formed due to
(A) Gain of electrons
(B) Loss of protons
(C) Loss of electrons
(D) Gain of protons

24. Match the following molar masses and choose the answer from the code given below
              Column I                         Column II
(i) Sulphuric acid H2SO4          (A) 78g/mol
(ii) Benzene C6H6                    (B) 164g/mol
(iii) Sodium Chloride NaCl       (C) 98g/mol
(iv) Calcium nitrate                   (d) 58.5g/mol
[Atomic mass of: Ca = 40u, H = 1u, S = 32u, C = 12u, Na = 23u, N = 14u, O = 16u, Cl = 35.5u]
(A) (i) -C, (ii) -A, (iii) -D, (iv) -B
(B) (i) -B, (ii) -A, (iii) -D, (iv) -C
(C) (i) -C, (ii) -A, (iii) -B, (iv) -D
(D) (i) -A, (ii) -D, (iii) -C, (iv) -B

25. 144 grams of pure water is decomposed by passing electricity. 16 grams of hydrogen and 128 grams of oxygen are obtained. Which chemical law is illustrated by this statement?

26. What is the atomicity of -
(a) Ozone
(b) Nitrogen

27. What is the atomicity of -
(a) Neon
(b) Sulphur

28. What name is given to the number 6.023 x 1023?

29. Select the correct statement and choose the answer from the code given below-
I. Gram molecular mass of a substance contains 6.022X1023molecules.
II. The molar mass of a substance is the mass of 1 mole of that substance.
III. Gram molecular mass of H2O is 1.
IV. The molar mass of nitrogen molecule is 28g/mol
(A) I and III
(B) I, II and IV
(C) Only II
(D) All of these

30. Which of the following are ionic compounds?
(i) NH4+ (ii) CaCl2 (iii) C6H6 (iv) Al2O3
(A) (i) and (ii)
(B) (ii) and (iv)
(C) (i) and (iii)
(D) None of the above

31. Select the correct statement and choose the answer from the code given below
(i) All metals form cations by gaining electrons
(ii) All metals form cations by losing electrons
(iii) Hydrogen is a non metal but forms a cation
(iv) All nonmetals form anions
(A) (ii) only
(B) (iii) and (iv)
(C) (i) and (iii)
(D) (ii) , (iii) and (iv)

32. An atom is made up of negatively charged electrons and positively charged protons, still it is electrically neutral. The reason is
(A) The electrons and protons are equal in number so no net charge is present.
(B) The neutrons neutralize the charge and make the atom neutral
(C) The charge on the atom is too small to be detected
(D) Both (1) and (2)

33. When calcium metal (Ca) changes to calcium ion (Ca+2) the number of electron, proton andneutron
(A) Remains same
(B) Number of neutrons remains the same while the number of both protons and electrons changes.
(C) Number of neutrons and protons remains same while the number of electrons decreases by 2.
(D) Number of neutrons and electrons remains the same while number of protons increases by 2

34. What is the contradiction against Dalton's atomic theory in the formula of sucrose, C^22^u ?

35. Select the correct statement and choose the answer from the code given below

I.One mole of all substances in gaseous state contains equal number of particles (6.022X1023) and occupy equal volume (22.4 L) at STP.
II.One mole of all substances contains equal number of particles (6.022X1023) at all temperatures and pressure.
III.Avogadro number is equal to 6.022X1023 particles (atoms, molecules,ions) .
IV.One mole of N2 does not contain same number of molecules as in one mole of H2.
(A) I and IV
(B) I and III
(C) I, III and IV
(D) None of these

36. Write the postulate of Dalton's atomic theory that eplains the Law of definite proportions.

37. The number of atoms present in 0.4 mole of Calcium (Ca) are
(A) 2.408 X 1023
(B) 24.08 X 1023
(C) 2.408 X 1024
(D) 6.022 X 1023

38. An element A forms an oxide A2O3, the valency of A is
(A) 2
(B) 3
(C) 5
(D) 1

39. Formula mass of ammonium sulphate (NH4) 2SO4 is (N=14, H=1, O=16 and S=32)
(A) 114u
(B) 132u
(C) 120u
(D) 132u

40. The chemical formula of Phosphorus and Sulphur are respectively:
(A) P4 and S
(B) P3 and S8
(C) P4 and S8
(D) P4 and S6

41. Which of the following gases can exist in atomic form?
(A) Oxygen (O)
(B) Neon (Ne)
(C) Hydrogen (H)
(D) Nitrogen(N)

42. Define 'Atomic mass unit' or what is '1 «' ?

43. Write the formula of 'Limestone'. Name the elements present in it.

44. What is the molar mass of a sulphur molecule ?

45. What is Latin name of 'silver' ? What is its symbol ?

46. Write the chemical symbols of -
(a) Silver
(b) Potassium

47. Match the correct formula and choose the answer from the code given below
(i) Hydrogen sulphide              (A) CuO
(ii) Ammonia                            (B) Al2O3
(iii) Aluminium oxide                (C) H2S
(iv) Cupric Oxide                     (D) NH3
(A) (i) -C,(ii) -A,(iii) -D,(iv) -B
(B) (i) -A, (ii) -D, (iii) - C, (iv) -B
(C) (i) -C, (ii) -D, (iii) -B, (iv) -A
(D) (i) -B, (ii) -D, (iii) -A, (iv) -C

48. Calculate the mass of one amu in kilograms. .

49. Ratio of nitrogen and hydrogen in ammonia (NH3) by mass is:
(A) 3 : 1
(B) 1 : 3
(C) 3 :14
(D) 14 : 3

50. Which of the following statement is correct? Choose the answer from the code given below.
I Dalton proposed the term Atom
II.Maharishi Kanad proposed the term Atom
III.Atom was considered to be indivisible by Dalton
IV. Kanad said Atom could be further divided
(A) I only
(B) II and IV
(C) I and III
(D) All of these

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