CBSE Class 9 Chemistry Atoms And Molecules Worksheet Set A

Exam Questions NCERT Class 9 Science Chapter 3 Atoms And Molecules

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–).

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. 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.

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 SO₄^2- 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