Reversing the calculations
2 marks · EasyNitric acid reacts with sodium carbonate according to the following equation:
Calculate the minimum volume, in cm³, of 0.250 mol/dm³ nitric acid solution needed to completely react with 25.0 cm³ of 0.100 mol/dm³ sodium carbonate solution.
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20.0 cm³ (±1)
Common mistakes: Incorrect reaction ratio; Not converting between dm³ and cm³
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Marking points
M1 · 1 mark · Calculates moles of sodium carbonate: 0.100 × 25.0/1000 = 0.00250 mol. Uses 1:2 ratio to calculate required moles of nitric acid: 0.00250 × 2 = 0.00500 mol.
A1 · 1 mark · Calculates volume: 0.00500 / 0.250 = 0.0200 dm³ = 20.0 cm³.
Worked solution
Moles Na₂CO₃ = 0.100 × 25.0/1000 = 0.00250 mol
Moles HNO₃ needed = 0.00250 × 2 = 0.00500 mol
Volume (dm³) = 0.00500 / 0.250 = 0.0200 dm³
Volume (cm³) = 0.0200 × 1000 = 20.0 cm³