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Mean sodium ion concentration
The student calculated the concentration of sodium chloride in the salt solution.
| Trial 1 | Trial 2 | Trial 3 | Trial 4 |
|---|---|---|---|
| 35.2 | 34.6 | 36.4 | 33.8 |
The percentage by mass of sodium ions in sodium chloride is 39.3%.
Calculate the mean concentration of sodium ions in the salt solution.
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Marking points
- 1 Finds the mean of the sodium chloride concentrations using the supplied trials.
- 2 Obtains 35.0 g/dm³ for the mean sodium chloride concentration, or a valid corresponding intermediate under the official alternative routes.
- 3 Applies the sodium mass percentage by multiplying by 39.3/100.
- 4 Gives a correctly rounded final mean sodium-ion concentration of 13.8 g/dm³ (or an official error-carried-forward equivalent).
Full-mark answer
Mean sodium chloride concentration = (35.2 + 34.6 + 36.4 + 33.8) ÷ 4 = 35.0 g/dm³. Mean sodium-ion concentration = 35.0 × 39.3/100 = 13.755 g/dm³ = 13.8 g/dm³ (3 s.f.).
Why this answer loses marks
The student calculates the mean salt concentration and stops there.
The question asks for the sodium-ion concentration, so the stated percentage composition must still be applied to the mean.