1.15 g of sodium completely reacts with water at standard temperature and pressure (STP). What volume of hydrogen at STP is produced by this reaction? [Assume in this question that 1 mole of any gas at STP has a volume of 22.4 dm3 (litres).] [Ar values: H = 1, 0 = 16, Na = 23]

Answer:

10) B

We must first write down the full equation for the reaction.

2Na + 2H2O -> 2NaOH + H2

Moles of Na = m/Mr = (1.15 ÷ 23) ≈ (1.2 ÷ 24) = 0.05 mol
Molar ratio (NaOH:H2) = 2:1
Moles of H2 = (0.05÷2) = 0.025mol

We know 1 mol gives 22.4dm³ of gas
0.025 mol = 22.4 x 0.025 = 0.56 dm³
1dm³ = 1000cm³
0.56 dm³ = 560cm³

The answer is B