Using the molar volume in calculations with chemical equations
3 marks · EasyPotassium reacts with water to produce potassium hydroxide and hydrogen gas according to:
If 4.6 g of potassium (Ar: K = 39) reacts completely, calculate:
the number of moles of potassium reacted.
the number of moles of hydrogen gas formed.
the volume of hydrogen gas produced at rtp (in dm³).
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(i) 0.118 mol
(ii) 0.059 mol
(iii) 1.42 dm³
Common mistakes: Using wrong molar ratio; Incorrect molecular mass
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Marking points
M1 (a(i)) · 1 mark · Correctly calculates moles of K: 4.6 / 39 = 0.118 (mol)
M2 (a(ii)) · 1 mark · Uses 2:1 mole ratio to find moles of H2: 0.118 / 2 = 0.059 mol (follow-through allowed) (follow-through)
A1 (a(iii)) · 1 mark · Finds volume: 0.059 × 24 = 1.42 dm³ (or ecf from earlier working) (follow-through allowed) (follow-through)
Worked solution
(i) Number of moles of K = 4.6 / 39 = 0.118 mol
(ii) From the equation, 2 mol K makes 1 mol H₂, so 0.118 / 2 = 0.059 mol H₂ formed
(iii) Volume of H₂ = 0.059 × 24 = 1.42 dm³