NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules


What are polyatomic ions? Give examples.


Polyatomic ions are the group of atoms carrying a net charge on them. This charge may be positive or negative.


Ammonium ion (NH4+), Nitrate ion (NO3), Carbonate ion (CO32–), Sulphate ion (SO42–)


Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.



What is the mass of:

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?



Convert into mole:

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide



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) ?



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.



Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.

(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)



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


(a) Gram atomic mass of carbon = 12 g

Gram atomic mass of hydrogen = 1g

Molar mass of ethyne (C2H2) = 2 x 12 + 2 x 1

 = 26 g/mol

(b) Gram atomic mass of sulphur = 32 g

Molar mass of sulphur molecule (S8) = 8 x 32

        = 256 g/mol

(c) Gram atomic mass of phosphorus = 31 g

Molar mass of phosphorus molecule (P4) = 4 x 31

        = 124 g/mol

(d) Gram atomic mass of hydrogen = 1g

Gram atomic mass of chlorine = 35.5 g

Molar mass of hydrochloric acid (HCl) = 1 + 35.5

         = 36.5 g/mol

(e) Gram atomic mass of hydrogen = 1g

Gram atomic mass of nitrogen = 14 g

Gram atomic mass of oxygen = 16 g

Molar mass of nitric acid (HNO3) = 1 + 14 + 3 x 16

  = 63 g/mol


Give the names of the elements present in the following compounds.

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate.




Elements present


Quick lime (CaO)

Carbon, Oxygen


Hydrogen bromide (HBr)

Hydrogen, Bromine


Baking powder (NaHCO3)

Sodium, Hydrogen, Carbon, Oxygen


Potassium sulphate (K2SO4)

Potassium, Sulphur, Oxygen


Write the chemical formulae of the following.

(a) Magnesium chloride

(b) Calcium oxide

(c) Copper nitrate

(d) Aluminium chloride

(e) Calcium carbonate


(a) Magnesium chloride: MgCl2

(b) Calcium oxide: CaO

(c) Copper nitrate: Cu(NO3)2

(d) Aluminium chloride: AlCl3

(e) Calcium carbonate: CaCO3


When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00g 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?


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