Reversing the calculations
2 marks · NormalAmmonia reacts with hydrochloric acid according to the equation:
Calculate the required volume (in cm³) of 0.250 mol/dm³ ammonia solution needed to react completely with 15.0 cm³ of 0.200 mol/dm³ hydrochloric acid solution.
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12.0 cm³ (±0.1)
Also accept: 11.9 cm³; 12 cm³
Common mistakes: Mixing up volumes and concentrations; Forgetting unit conversions
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Marking points
M1 · 1 mark · Correctly calculates moles of HCl using concentration and volume.
A1 · 1 mark · Correctly uses stoichiometry and calculation to find volume of ammonia solution with correct unit.
Worked solution
Number of moles of HCl = (15.0 × 0.200) / 1000 = 0.00300 mol
From the equation, NH3 and HCl react 1:1, so moles of NH3 needed = 0.00300 mol.
Volume of NH3 solution = moles / concentration = 0.00300 / 0.250 = 0.0120 dm³ = 12.0 cm³