A polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, Nitrate (NO₃^-) , hydroxide ion (OH - )

(a) Magnesium chloride

→ MgCl₂

(b) Calcium oxide

→ CaO

(c) Copper nitrate

→ Cu (NO₃)₂

(d) Aluminium chloride

→ AlCl₃

(e) Calcium carbonate

→ CaCO₃

(a) Quick lime
→ Calcium and oxygen

(b) Hydrogen bromide
→ Hydrogen and bromine

(c) Baking powder
→ Sodium, hydrogen, carbon, and oxygen

(d) Potassium sulphate
→ Potassium, sulphur, and oxygen

(a) Ethyne, C₂H₂
→ Molar mass of ethyne, C₂H₂ = 2 × 12 + 2 × 1 = 26 g

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

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

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

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

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)?

Answer

(a) The mass of 1 mole of nitrogen atoms is 14 g.
 

(b) The mass of 4 moles of aluminium atoms is (4 × 27) g = 108 g

(c) The mass of 10 moles of sodium sulphite (Na₂SO₃) is
10 × [2 × 23 + 32 + 3 × 16] g = 10 × 126 g = 1260 g

Convert into mole.

(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide

Answer

(a) 32 g of oxygen gas = 1 mole
Then, 12 g of oxygen gas = 12 / 32 mole = 0.375 mole

(b) 18 g of water = 1 mole
Then, 20 g of water = 20 / 18 mole = 1.111 mole

(c) 44 g of carbon dioxide = 1 mole
Then, 22 g of carbon dioxide = 22 / 44 mole = 0.5 mole

What is the mass of:

(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?

Answer

(a) Mass of one mole of oxygen atoms = 16 g
Then, mass of 0.2 mole of oxygen atoms = 0.2 × 16g = 3.2 g

(b) Mass of one mole of water molecule = 18 g

Then, mass of 0.5 mole of water molecules = 0.5 × 18 g = 9 g

1 mole of solid sulphur (S8) = 8 × 32 g = 256 g
i.e., 256 g of solid sulphur contains = 6.022 × 10²³ molecules
Then, 16 g of solid sulphur contains = 6.022 × 10²³ / 256  = 16 molecules
= 3.76375 × 10²² molecules