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Finding the isotonic concentration
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 |
Determine the concentration of salt in solution Z. Use Figure 2.
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Marking points
- 1 Reads the zero-percentage-change intercept from the line of best fit; without a drawn line, gives a value from 0.64 to 0.66 mol/dm³.
Full-mark answer
The line of best fit crosses 0% change at about 0.65 mol/dm³, so the concentration of solution Z is approximately 0.65 mol/dm³ (accept 0.64 to 0.66 mol/dm³ if no line is shown).
Why this answer loses marks
Choose the nearest measured concentration from the table.
The unknown comes from where the fitted trend crosses zero mass change, followed by a reading from the horizontal axis.