Question 18M.3.sl.TZ1.2d.i
| Date | May 2018 | Marks available | [Maximum mark: 1] | Reference code | 18M.3.sl.TZ1.2d.i |
| Level | sl | Paper | 3 | Time zone | TZ1 |
| Command term | Justify | Question number | d.i | Adapted from | N/A |
Students were asked to investigate how a change in concentration of hydrochloric acid, HCl, affects the initial rate of its reaction with marble chips, CaCO3.
They decided to measure how long the reaction took to complete when similar chips were added to 50.0 cm3 of 1.00 mol dm−3 acid and 50.0 cm3 of 2.00 mol dm−3 acid.
Two methods were proposed:
(1) using small chips, keeping the acid in excess, and recording the time taken for the solid to disappear
(2) using large chips, keeping the marble in excess, and recording the time taken for bubbles to stop forming.
A group recorded the following results with 1.00 mol dm−3 hydrochloric acid:
Justify why it is inappropriate to record the uncertainty of the mean as ±0.01 s.
[1]
variation of individual values is much greater «than this uncertainty»
OR
«uncertainty» does not take into account «student» reaction time
[1 mark]
