Find the:

(a) number of moles in 10 g of CaCO₃,

(b) weight of 0.2 mole of H₂ gas,

(c) number of molecules in 3.2 g of SO₂.

(a) Number of moles in 10 g of CaCO₃ —

Mass of 1 mole of CaCO₃ = 40 + 12 + 3(16) = 52 + 48 = 100 g

100 g of CaCO₃ = 1 mole

∴ 10 g of CaCO₃ = $\dfrac{1 \times 10}{100}$

= 0.1 mole

(b) Weight of 0.2 mole H₂ gas —

1 mole of H₂ weighs = 2 g

∴ 0.2 moles will weigh = $\dfrac{2 \times 0.2}{1}$ = 0.4 g

(c) No. of molecules in 3.2 g of SO₂ —

Molecular wt. of any substance contain 6 × 10²³ molecules.

Mass of 1 mole of SO₂ is 32 + 2(16) = 32 + 32 = 64 g

64 g of SO₂ contains 6 × 10²³ molecules

∴ 3.2 g of SO₂ contains $\dfrac{6 \times 10^{23} \times 3.2 }{64}$

= 3 x 10²² molecules.