ATOMS

Q 1: What are polyatomic ions? Give examples?

Ans: A polyatomic ion is a group of atoms carrying a charge (positive or negative). 

(OH−) hydroxide ion 

carbonate ion,

sulphate ion.

Q 2: Give the names of the elements present in the following compounds:

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate

Ans:

Compound	Chemical formula	Elements present
Quick lime	CaO	Calcium, oxygen
Hydrogen bromide	HBr	Hydrogen, bromine
Baking powder	NaHCO3	Sodium, hydrogen, carbon, oxygen
Potassium sulphate	K2SO4	Potassium, sulphur, oxygen

Q 3: 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

Ans:

(a) Molar mass of ethyne,                     C₂H₂ = 2 × 12 + 2 × 1 = 28 g

(b) Molar mass of sulphur molecule,         S₈ = 8 × 32 = 256 g

(c) Molar mass of phosphorus molecule,  P₄ = 4 × 31 = 124 g

(d) Molar mass of hydrochloric acid,        HCl = 1 + 35.5 = 36.5 g

(e) Molar mass of nitric acid,                HNO₃ = 1 + 14 + 3 × 16 = 63 g

Q 4: Define atomic mass unit.

Ans: Mass unit equal to exactly one-twelfth 1/12^th the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.