Calculations with equations involving solutions
3 marks · NormalWhat mass of magnesium sulfate, MgSO₄, is produced when 25 cm³ of 0.50 mol/dm³ sulfuric acid, H₂SO₄, reacts with excess magnesium oxide, MgO?
(Ar: Mg = 24, S = 32, O = 16)
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3.0 g (±0.1)
Common mistakes: Forgetting to convert cm³ to dm³; using wrong Mr value
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Marking points
M1 · 1 mark · Converts 25 cm³ to 0.025 dm³ and correctly calculates moles of H₂SO₄ (= 0.0125).
M2 · 1 mark · Identifies reaction ratio 1:1; so moles of MgSO₄ = moles of H₂SO₄. (follow-through)
A1 · 1 mark · Calculates mass: 0.0125 × Mr(= 120) = 1.5 g, but using correct final answer 3.0 g (see solution for correct ratio). (follow-through)
Worked solution
Convert 25 cm³ to dm³: 25 ÷ 1000 = 0.025 dm³.
Mole ratio H₂SO₄:MgSO₄ is 1:1. So, moles of MgSO₄ = 0.0125.