Some students design an experiment to investigate the effect of hydrochloric acid (HClHClHCl) concentration on the rate of reaction with calcium carbonate. The apparatus collected carbon dioxide gas in a gas syringe.
Method:
Some of the results are shown in the tables below:
Table 1: Results for 0.5 mol dm−30.5\text{ mol dm}^{-3}0.5 mol dm−3 HCl
| Time (s) | Trial 1 (cm3\text{cm}^3cm3) | Trial 2 (cm3\text{cm}^3cm3) | Trial 3 (cm3\text{cm}^3cm3) | Mean (cm3\text{cm}^3cm3) | Standard deviation |
|---|---|---|---|---|---|
| 15 | 8 | 9 | 8 | 8.3 | 0.6 |
| 30 | 15 | 17 | 16 | 16.0 | 1.0 |
| 45 | 22 | 24 | 23 | 23.0 | 1.0 |
| 60 | 29 | 31 | 30 | 30.0 | 1.0 |
Table 2: Results for 2.5 mol dm−32.5\text{ mol dm}^{-3}2.5 mol dm−3 HCl
| Time (s) | Trial 1 (cm3\text{cm}^3cm3) | Trial 2 (cm3\text{cm}^3cm3) | Trial 3 (cm3\text{cm}^3cm3) | Mean (cm3\text{cm}^3cm3) | Standard deviation |
|---|---|---|---|---|---|
| 15 | 25 | 22 | 25 | 24.0 | 1.7 |
| 30 | 45 | 41 | 46 | 44.0 | [ To be calculated ] |
| 45 | 58 | 54 | 56 | 56.0 | 2.0 |
| 60 | 67 | 61 | 64 | 64.0 | 3.0 |
(i) State the independent variable in this investigation.
(ii) Describe how a student could produce a 1.5 mol dm−31.5\text{ mol dm}^{-3}1.5 mol dm−3 dilute hydrochloric acid solution from the 2.5 mol dm−32.5\text{ mol dm}^{-3}2.5 mol dm−3 stock solution.
(i) Calculate the standard deviation for the 30 s30\text{ s}30 s result at 2.5 mol dm−32.5\text{ mol dm}^{-3}2.5 mol dm−3 HCl. Use the formula:
s=∑(x−xˉ)2n−1 s = \sqrt{\frac{\sum(x - \bar{x})^2}{n - 1}} s=n−1∑(x−xˉ)2Give your answer to 1 decimal place.
(ii) Calculate the initial rate of reaction over the first 15 s15\text{ s}15 s for the result at 0.5 mol dm−30.5\text{ mol dm}^{-3}0.5 mol dm−3 HCl. Include the appropriate unit.
