# CBSE Class 9 Science Atoms and Molecules assignment

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## Atoms And Molecules Class 9 Science Assignment Pdf

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

### Class 9 Science Assignment for Atoms And Molecules

Question : In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + ethanoic acid →sodium ethanoate + carbon + dioxide + water

Solution : Law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction.
It means the mass remains the same. So, we add the mass of the reactants on LHS and add the mass of all products on RHS
LHS = 5.3 g + 6 g = 11.3 g
RHS = 8.2g + 2.2g + 0.9g = 11.3g
LHS = RHS
So, the observations are in agreement with the law of conservation of mass.

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?
Solution:'Law of constant proportions' states that composition of a compound is always fixed.
Applying this
∵1 g of hydrogen gas combines with oxygen = 8 g
∴3 g of hydrogen gas will combine with oxygen = 8 x 3 = 24 g

Question : Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?
Solution: Following postulate of Dalton's atomic theory is the result of the law of conservation of mass. 'Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.'

Question : Which postulate of Dalton's atomic theory can explain the law of definite proportions?
Solution: Following postulate of Dalton's atomic theory can explain the 'law of definite proportions'.
'The relative number and kinds of atoms are constant in a given compound.'

Question : Define the atomic mass unit.
Solution: One atomic mass unit (amu) is a mass unit equal to exactly onetwelfth (1/12th) the mass of one atom of carbon-12.' The relative atomic
masses of all the elements have been found with respect to an atom of carbon-12.

Question : Why is it not possible to see an atom with naked eyes?
Solution : As an atom is extremely small in size, it is not possible to see it with naked eyes.
Generally radius of an atom is of the order of nanometres. For example, atomic radius of hydrogen atom is 10-10m (or 10-1nm).

Question : Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium sulphide
(iv) magnesium hydroxide
Solution:
(i)Na2O
(ii)NH4Cl
(iii)Na2SO4
(iv)Mg(OH)2

Question : Write down the names of compounds represented by the following formulae.
(i) AL2(SO4)3 (ii) CaCL2 (iii) K2SO4 (iv) KNO3(v) CaCO3
Solution:
(i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate.

Question : What is meant by the term chemical formula?
Solution:  Chemical formula of a compound (or element) is the symbolic representation of its composition. It represents
(i) The number and kind of atoms present per molecule of the compound,
(ii) One mole of the compound,
(iii) Molar mass of the compound.

Question : How many atoms are present in a
(i) H2S molecule and (ii) PO43−ion?
Solution:
(i) 2 atom of hydrogen + 1 atom of sulphur = three (3) atoms (in a H2S molecule).
(ii) 1 atom of phosphorus + 4 atoms of oxygen = five (5) atoms (in aPO43−ion).

Question : Calculate the molecular masses of
H2, O2, CL2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Solution:
(i) Molecular mass of H2(hydrogen)
= Atomic mass of hydrogen × 2
= 1 × 2 = 2 u

(ii) Molecular mass of O2 (oxygen)
=Atomic mass of oxygen × 2
= 16 × 2 = 32 u

(iii) Molecular mass of CI2(chlorine)
= Atomic mass of chlorine × 2
= 35.5 × 2 = 71 u

(iv) Molecular mass of CO2(carbon dioxide)
= (Atomic mass of carbon × 1)+ (Atomic mass of oxygen × 2)
= 12 + (16 × 2) = 12 + 32 = 44 u

(v) Molecular mass of CH4(methane)
= (Atomic mass of carbon × 1) + (Atomic mass of hydrogen × 4)
= 12 + (1 × 4) = 12 + 4 = 16 u

(vi) Molecular mass of C2H6(ethane)
= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 6)
= (12 × 2) + (1 × 6) = 24 + 6 = 30 u

(vii) Molecular mass of C2H4(ethene)
= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 4)
= (12 × 2) + (1 × 4) = 24 + 4 = 28 u

(viii) Molecular mass of NH3(ammonia)
= (Atomic mass of nitrogen × 1) + (Atomic mass of hydrogen × 3)
= (14 × 1) + (1 × 3) = 14 + 3 = 17 u

(ix) Molecular mass of CH3OH (methanol or methyl alcohol)
= (Atomic mass of carbon × 1) + (Atomic mass of hydrogen × 3)+ (Atomic mass of oxygen × 1) + (Atomic mass of hydrogen × 1)
= 12 + 3 + 16 + 1 = 32 u

Question : Calculate the formula unit masses of Zn0, Na20, K2C03.
Given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u
C = 12 u and 0 = 16 u.
Solution:
(i) Formula unit mass of ZnO (zinc oxide) =
65 + 16 = 81 u
(ii) Formula unit mass of Na2O (sodium oxide) =
(23 × 2) + (16 × 1) = 46 + 16 = 62 u
(iii) Formula unit mass of K2C03(potassium carbonate) =
(39 × 2) + (12 × 1) + (16 × 3) = 78 + 12 + 48 = 138 u

Question : If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?
Solution:
1 mole carbon atom = 6.022 x 1023 atoms
Molar atomic mass = 12 g
6.022 x 1023 carbon atoms weigh = 12 g

1 carbon atom weighs ----12/6.022 x 1023 = 1.99 x 10-23g

Question : Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
Solution:
Molar mass of sodium = 23 g
1 mole atom = 6.022 x 1023 atoms
23 g sodium contains = 6.022 x 1023 atoms
1 g sodium contains = 6.022 x 1023 atoms
23
100 g sodium contains = 6.022 x 1023 atoms
= 2.618 x 1024 atoms
By the above method or by formula we find number of atoms in 100 g
Fe;
Number of atoms of an element in a given mass

=  Given mass / Gram atomic mass 100g x Avogadro's number
= 100g / 56g x 6.022 x 1023
= 1.075 x 1024 atoms
Hence, 100 g of sodium has more number of atoms as compared to 100 g of iron.

### Exercises

Question : A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g .of oxygen.
Calculate the percentage composition of the compound by weight.
Solution:
Mass of the compound = 0.24 g
Mass of boron = 0.096 g
Mass of oxygen = 0.144 g
Mass of boron 0.096 g Alternative method
Percentage of oxygen =100 percentage of boron
=100 - 40 = 60%

Question : When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
Solution:
First we find the proportion of mass of carbon and oxygen in carbon dioxide.
In CO2 , C : O = 12 : 32 or 3 : 8
In other words, we can say that
12.00 g carbon reacts with oxygen = 32.00 g
3.00 g carbon will react with oxygen = 32 x 3 /12 = 8g

Therefore, 3.00 g of carbon will always react with 8.00 g of oxygen to form CO2 (11g), even if large amount (50.00 g) of oxygen is present.
This answer will be governed by ‘the law of constant proportions’.

Question : What are polyatomic ions? Give examples.
Solution: The group of atoms which carry a fixed charge (either positive or negative) on them and behave as ions are called polyatomic ions.
Example
(i) Carbonate ion (ii) Sulphate ion
(iii) Ammonium ion (iv) Phosphate ion

Question : Write the chemical formulae of the following.
(i) Magnesium chloride (ii) Calcium oxide
(iii) Copper nitrate (iv) Aluminium chloride
(v) Calcium carbonate
Solution:
(i)  Formula = MgCl2 (Magnesium chloride)
(ii)  Formula = CaO (Calcium oxide)
(iii) Formula = Cu(NO3)2 (Copper nitrate)
(iv) Formula = AICI3 (Aluminium Chloride)
(v)  Formula = CaCO3 (Calcium carbonate)

Question : Give the names of the elements present in the following compounds.
(a) Quick lime (b) Hydrogen bromide
(c) Baking powder (d) Potassium sulphate
Solution:
(a) Quick lime- Calcium oxide - CaO
Elements–Calcium, oxygen.
(b)Hydrogen bromide- HBr
Elements- Hydrogen, bromine.
(c) Baking powder- Sodium hydrogen carbonate -NaHCO3
Elements- Sodium, hydrogen, carbon, oxygen.
(d)Potassium sulphate- K2SO4
Elements- Potassium, sulphur, oxygen.

Question : Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4(Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
Solution:
(a) Molar mass of C2H2
= (2 × Atomic mass of C) + (2 × Atomic mass of H)
= (2 × 12) + (2 × 1)
= 26 u
(b) Molar mass of S8
= (8 × Atomic mass of S)
= 8 × 32 = 256 u
(c) Molar mass of P4
= 4 × Atomic mass of P
= 4 × 31 = 124u
(d) Molar mass of HCI
= Atomic mass of hydrogen + Atomic mass of CI
= 1 + 35.5 = 36.5 u
(e) Molar mass of HN03
= Atomic mass of H + Atomic mass of N + (3 × Atomic mass of 0)
= 1 + 14 + (3 × 16) = 15 + 48 = 63 u

Question : What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2SO3)?
Solution:
(a) Molar mass of N atom = Atomic mass of N.
Mass of 1 mol of N atoms = 14 g
(b) Mass of 1 mole AI atoms = 27 g
Mass of 4 moles of AI atoms = 27 × 4 = 108 g
(c) Mass of 1 mole of Na2SO3 = (23 ×2 ) + 32 + (16 × 3)
= 46 + 32 + 48 = 126 g
Mass of 10 moles of Na2SO3 = 126 × 10 = 1260 g

Question : Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide.
Solution: Question : What is the mass of
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Solution:
(a) Mass of 1 mole O-atoms = 16 g
Mass of 0.2 mole O-atoms = 16 × 0.2 = 3.2 g
(b) Mass of 1 mole of H2O molecules = 18 g
Mass of 0.5 mole of H2O molecules = 18 × 0.5 = 9.0 g

Question : Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur
Solution:
Molar mass of sulphur (S8 ) = 32 x 8 = 256 g
Number of S8 molecules in 256 g of solid sulphur = 6.022 x 1023
Number of S8 molecules in 16 g of solid sulphur=6.022 x1023x 16g 256g
=3.76 x 1023 molecules

Question : Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
[HintThe mass of an ion is the same as that of an atom of the same element. Atomic mass of AI= 27 u.]
Solution: Question : 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?

Question : What do you mean by Molar Mass ?

Question : What is atomic mass unit ?

Question : What are molecules?

Question : What name is given to the number 6.023 x 1023?

Question : Name the building blocks of matter.

Question : How many metres are there in 1 nanometer (nm)?

Question : Define atomicity.

Question : Write the chemical formula of glucose.

Question : Which subatomic particle was not present in Thomson‘s model of the atom?

Question : Write the symbol of element lead and iron.

Question : Write down the formulae of aluminium sulphate and ethanol.

Question : What is the atomicity of –
(a) Ozone (b) Nitrogen
(c) Neon (d) Sulphur

Question : State main points of Dalton atomic theory.

Question : State the law of constant proportion.

Question : state the law of conservation of mass

Question : If 90g of pure water is obtained through 80g of oxygen and Xg of hydrogen. a. Find the value of X. b. Which chemical law is illustrated by this statement?

Question : Calculate the molecular mass of the following: a. H2O b. HCl c. H2S

Question : If 6g of hydrogen combine with 48g of oxygen, how many grams of water is obtained?

Question : If carbon and oxygen combine in a definite ratio of 3:8 to form carbon dioxide. What mass of oxygen gas is required in carbon dioxide with 10 g of carbon?

Question : Give symbol of the followinga. oxygen b. hydrogen c. carbon

Question : If atomic mass of carbon is 1u then what is the atomic mass of nitrogen (N=14, C=12)?

Question : Identify the anions, cations and charge present on them in following molecules: HCl , NaCl , H2O , Al2O3

Question : Write the chemical formula of the following:

a. Oxygen

b. Carbon dioxide

c. Ammonia

d. Calcium carbonate

Exam Questions NCERT Class 9 Science Chapter 3 Atoms And Molecules

Question. Define law of conservation of mass.
Ans : Law of conservation of mass states that, ‘Mass is neither created nor destroyed in a chemical reaction.’

Question. Name the scientists whose experimentation established laws of chemical combination. Name the laws also.
Ans : Antoine Laurent Lavoisier and Joseph L. Proust experimented and established two laws of chemical combination. These laws are : (i) Law of conservation of mass, (ii) Law of constant proportions.

Question. Give one example each of (i) Monovalent cation, (ii) Bivalent cation, (iii) Monovalent anion, (iv) Bivalent anion.
Ans : (i) K+ or Na+
(ii) Mg+2 or Ca+2
(iii) F– or Cl–
(iv) O2– or S2–

Question. How can Dalton’s atomic theory explain the Law of Constant Proportions?
Ans : According to Dalton’s atomic theory, atoms of the same elements are same. Also atoms combine in whole number. This means that the atoms can combine with each other in a simple fixed ratio to form molecules.

Question. What is ion?
Ans : An ion is a charged particle. It can be positive or negative.

Question. What do you mean by symbols of elements?
Ans : Each element is represented by a letter or group of two letters to write the chemical reactions conveniently. It is called symbol.

Question. How did the scientist lay the foundation of chemical sciences? Name the scientist.
Ans : Antoine Laurent Lavoisier laid the foundation of chemical science by establishing two important laws of chemical combination.

Question. An element has 8 electrons in its valence shell. What is its general name?
Ans : Noble gas.

Question. Write the names of the following compounds :
(a) NiS
(b) Mg(NO3)2
(c) Na2SO4
(d) Al(NO3)3
(e) K3PO4 (f) Ca3N2

Ans : (a) Nickel sulphide,
(b) Magnesium nitrate,
(c) Sodium sulphate,
(d) Aluminium nitrate,
(e) Potassium phosphate,
(f) Calcium nitride.

Question Give the derivation source of symbol of sodium (Na).
Ans : The symbol of ‘Na’ for sodium is derived from its Latin name ‘Natrium’.

Question. Give the full form of amu?
Ans : The full from of amu is atomic mass unit.

Question. Write the symbols of tungsten and iron.
Ans : (i) Tungsten (W) and
(ii) Iron (Fe)

Question. All elements have charged valency. Explain.
Ans : No, all elements do not form ions thus they, do not have a charge.

For example : Carbon has a valency of 4 and nitrogen has a valency of 3. Non-metals are formed without a charged valency. Example : In carbon tetrachloride, carbon has valency of 4 and chlorine has a valency of 1.

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.

Please refer to attached file forCBSE Class 9 Atoms and Molecules assignment

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