The volume occupied by 0.4 g of hydrogen gas at s.t.p. is [H = 1. 00; Molar volume at s.t.p. = 22.4 dm3]

Analyze the question: We need the volume of 0.4 g of hydrogen gas at s.t.p. The key point is that hydrogen gas is diatomic (H₂), so we use its molar mass, not the atomic mass.

Concept: At s.t.p., 1 mole of any gas occupies 22.4 dm³.

• Moles = mass ÷ molar mass
• Volume = moles × 22.4 dm³

Step-by-step solution:

1. Molar mass of H₂ = 2 × 1.00 = 2.00 g/mol

2. Number of moles = 0.4 g ÷ 2.00 g/mol = 0.2 mol

3. Volume at s.t.p. = 0.2 × 22.4 dm³ = 4.48 dm³

Check the answer against the options:

• A (2.24 dm³) would be 0.1 mol, which is 0.2 g of H₂.
• C (22.4 dm³) would be 1 mol, which is 2 g of H₂.
• D (44.8 dm³) would be 2 mol, which is 4 g of H₂.

Tip: Always check whether a gas is diatomic (H₂, O₂, N₂, Cl₂, F₂) before calculating its molar mass. A common mistake is using 1 g/mol for hydrogen, which gives 0.4 mol and 8.96 dm³, an answer that isn't among the options. Also, a quick shortcut: 2 g of H₂ occupies 22.4 dm³, so 0.4 g is one-fifth of that, giving 22.4 ÷ 5 = 4.48 dm³.