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Calculating percentage mass change
A student investigated the concentration of salt in solution Z using osmosis. Table 1 shows the results.
| Concentration of salt solution in mol/dm³ | Mass of tube at start in grams | Mass of tube after 2 hours in grams | Change in mass in grams | Percentage (%) change in mass |
|---|---|---|---|---|
| 0.0 | 15.54 | 16.50 | 0.96 | X |
| 0.2 | 15.16 | 15.78 | 0.62 | 4.1 |
| 0.4 | 15.00 | 15.35 | 0.35 | 2.3 |
| 0.6 | 15.29 | 15.37 | 0.08 | 0.5 |
| 0.8 | 14.95 | 14.75 | −0.20 | −1.3 |
| 1.0 | 14.77 | 14.40 | −0.37 | −2.5 |
Calculate value X in Table 1. Give your answer to 1 decimal place.
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Marking points
- 1 Divides the change in mass (0.96 g) by the starting mass (15.54 g) and multiplies by 100.
- 2 Obtains an unrounded percentage of approximately 6.1776%.
- 3 Gives 6.2% to 1 decimal place.
Full-mark answer
Percentage change = (0.96 ÷ 15.54) × 100 = 6.1776…%, so X = 6.2% to 1 decimal place.
Why this answer loses marks
Divide the change by the final mass, then convert it to a percentage.
Percentage change uses the starting mass as the denominator, not the final mass.