Question 18M.3.sl.TZ1.2d.iii
| Date | May 2018 | Marks available | [Maximum mark: 1] | Reference code | 18M.3.sl.TZ1.2d.iii |
| Level | sl | Paper | 3 | Time zone | TZ1 |
| Command term | Justify, State | Question number | d.iii | 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:
Another student, working alone, always dropped the marble chips into the acid and then picked up the stopwatch to start it. State, giving a reason, whether this introduced a random or systematic error.
[1]
systematic AND always makes the time shorter «than the actual value»
OR
systematic AND it is an error in the method used «not an individual measurement»
OR
systematic AND more repetitions would not reduce the error
Accept, as reason, “it always affects the value in the same direction” OR “the error is consistent”.
[1 mark]
