Concentrations of solutions
3 marks · NormalCalculate the mass of sodium nitrate (NaNO₃, Ar: Na = 23, N = 14, O = 16) required to make 250 cm³ of a 0.050 mol/dm³ solution.
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1.06 g (±0.02)
Also accept: 1.1 g; 1.058 g
Common mistakes: Not converting 250 cm³ to 0.25 dm³; Using the Mr incorrectly
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Marking points
M1 · 1 mark · Converts 250 cm³ to 0.25 dm³ or uses correct approach: amount (mol) = concentration × volume
M2 · 1 mark · Correctly calculates amount of NaNO₃ needed: 0.050 × 0.25 = 0.0125 mol
A1 · 1 mark · Correctly calculates mass: 0.0125 × 85 = 1.06 g (accept rounding) (follow-through)
Worked solution
Volume = 250 cm³ = 0.25 dm³
Mr(NaNO₃) = 23 + 14 + (16 × 3) = 23 + 14 + 48 = 85