Question Constellation Sign up for free AQA · GCSE · Chemistry · Higher
Calculating a mean with uncertainty
A student investigated the energy change of the reaction between zinc and copper sulfate solution.
- Measure 25 cm³ of copper sulfate solution into a polystyrene cup.
- Measure the temperature of the copper sulfate solution.
- Add 0.20 g of zinc powder to the copper sulfate solution.
- Stir the reaction mixture.
- Record the highest temperature reached.
- Repeat steps 1 to 5 with different masses of zinc powder.
A different student repeated steps 1 to 5 of the method four times using 0.50 g of zinc powder.
| Trial 1 | Trial 2 | Trial 3 | Trial 4 | |
|---|---|---|---|---|
| Highest temperature reached in °C | 37.6 | 37.2 | 37.8 | 37.4 |
Calculate the mean highest temperature reached. Include the uncertainty in your answer.
Write your answer first. You can study the marking guidance whenever you need it.
Study the marking See what earns credit and compare it with a full-mark answer.
Marking points
- 1 Uses all four temperatures and divides their sum by four.
- 2 Mean value is 37.5 °C.
- 3 Final combined result is 37.5 °C ± 0.3 °C.
Full-mark answer
Mean = (37.6 + 37.2 + 37.8 + 37.4) ÷ 4 = 37.5 °C. Half the range = (37.8 − 37.2) ÷ 2 = 0.3 °C. Therefore the result is 37.5 ± 0.3 °C.
Why this answer loses marks
I used the full range as the uncertainty.
This result needs half the spread, paired with the calculated mean.