Calculations involving gas volumes
4 marks · NormalMagnesium reacts with steam to produce magnesium oxide and hydrogen gas as shown:
Calculate the number of moles of hydrogen gas produced when 2.40 g of magnesium completely reacts. (M_r(Mg) = 24)
What is the volume (in dm³ at rtp) of hydrogen gas formed?
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(a) 0.10 mol
(b) 2.4 dm³
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Marking points
M1 (a) · 1 mark · Calculates moles of magnesium: 2.40 ÷ 24 = 0.10 mol
A1 (a) · 1 mark · States that moles of H₂ produced = moles of Mg (1:1 ratio), so 0.10 mol (follow-through)
M2 (b) · 1 mark · Uses volume = moles × 24 (dm³) (follow-through)
A2 (b) · 1 mark · Correct answer: 2.4 dm³ (follow-through)
Worked solution
a) Moles of Mg = 2.40 g / 24 = 0.10 mol. So, 0.10 mol H₂ produced (1:1 ratio).
b) Volume = moles × molar volume = 0.10 × 24 = 2.4 dm³