Find the:

(a) number of molecules in 73 g of HCl,

(b) weight of 0.5 mole of O₂,

(c) number of molecules in 1.8 g of H₂O,

(d) number of moles in 10 g of CaCO₃,

(e) weight of 0.2 mole of H₂ gas,

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

(a) Number of molecules in 73 g of HCl —

Molecular wt. of any substance contain 6.022 × 10²³ molecules.

Mass of 1 mole of HCl is 1 + 35.5 = 36.5 g

36.5 g of HCl contains 6.022 × 10²³ molecules

∴ 73 g of HCl contains $\dfrac{6.022 \times 10^{23} \times 73 }{36.5}$

= 1.2 × 10²⁴ molecules

(b) Weight of 0.5 mole of O₂ —

1 mole of O₂ weighs = 2O = 2 x 16 = 32 g

∴ 0.5 moles will weigh = $\dfrac{32}{2}$ = 16 g

(c) Number of molecules in 1.8 g of H₂O —

Molecular wt. of any substance contains 6.022 × 10²³ molecules.

Mass of 1 mole of H₂O is (2 x 1) + 16 = 2 + 16 = 18 g

18 g of H₂O contains 6.022 × 10²³ molecules

∴ 1.8 g of H₂O contains $\dfrac{6.022 \times 10^{23} \times 1.8 }{18}$

= 6.02 × 10²² molecules

(d) 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

(e) 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

(f) 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.