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Evaluating methods and suggesting improvements

Edexcel IAL Chemistry · WCH13 Practical Skills in Chemistry I

The last part of many Unit 3 questions asks you to evaluate: what went wrong, how it affected the result, and how to improve it. Vague answers score nothing; specific, explained ones score well.

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Practise this lesson

A structure that scores

1. Identify the problem precisely (heat lost through the open top of the cup; gas escaping while the bung is replaced).

2. State the effect on the measured value and the result (ΔT too small, so ΔH less negative).

3. Suggest a specific improvement and say how it helps (fit a lid to reduce heat loss).

Avoid: 'human error', 'more accurate' with no reason, 'repeat the experiment' (unless the problem is random error), and 'be more careful'.

Improvements by experiment

Experiment	Typical improvements
Gas volume	add the solid without opening the flask; use a gas syringe for soluble gases; larger mass or 3 d.p. balance
Calorimetry	lid and insulation; readings every 30 s or a data logger; extrapolate; 0.1 °C thermometer for small changes
Titration	larger titre (more dilute burette solution); pipette instead of measuring cylinder; concordant repeats
Rates	water bath; light sensor instead of eye; start timing at mixing; more data points
Preparations	fewer transfers; rinse apparatus into the next stage; dry product fully before weighing

Which way will the result go?

Always work through the calculation:

Gas lost: V too small, so n(gas) too small, molar volume too small, and a molar mass calculated as mass / n too large.
Heat lost: ΔT too small, so ΔH less negative (smaller magnitude).
Product still wet: mass too large, % yield too high.
Burette not rinsed with its solution (diluted): titre too large.

If a single anomalous result sits in a clear pattern, you do not need to repeat the whole experiment.

Worked example

Burning 0.92 g of ethanol (M = 46.0) in a spirit burner heats 100 g of water by 20.0 °C. The data-book ΔcH is -1367 kJ mol^-1. Calculate ΔcH and the % error, and suggest two improvements.

q = 100 x 4.18 x 20.0 = 8360 J; n = 0.92 / 46.0 = 0.0200 mol
ΔcH = -8360 / 0.0200 = -418 kJ mol^-1
% error = (1367 - 418) / 1367 x 100 = 69%
Large heat losses and incomplete combustion: use a draught shield, put a lid on the calorimeter and keep the flame close to it.

Answer: -418 kJ mol^-1, 69% error

Key idea: Identify, state the effect, give a specific improvement with its reason. Work out which way each error moves the result. Do not write 'human error' or 'more accurate' alone.

Check yourself

Heat loss in a calorimetry experiment affects the calculated ΔH for an exothermic reaction how?
- It makes ΔH more negative
- It makes ΔH less negative (smaller in magnitude)
- It has no effect on ΔH
- It makes ΔH positive
Show answer
It makes ΔH less negative (smaller in magnitude). ΔT is too small, so q is too small.
A molar mass is found from mass / moles, where moles come from a gas volume. Some gas escapes. How is the molar mass affected?
- It is too low
- It is unchanged
- It becomes zero
- It is too high
Show answer
It is too high. Fewer moles of gas measured means mass / n is larger.
Which suggested improvement to a calorimetry experiment would gain credit?
- Be more careful
- Put a lid on the polystyrene cup to reduce heat loss
- Use more accurate equipment
- Repeat the experiment
Show answer
Put a lid on the polystyrene cup to reduce heat loss. It names the change and how it helps.
Five results show a clear trend, with one anomalous point. What should the student do?
- Repeat the whole experiment
- Include the anomaly in the mean
- Ignore the anomaly when drawing the line; no repeat is needed because the pattern is clear
- Change the independent variable
Show answer
Ignore the anomaly when drawing the line; no repeat is needed because the pattern is clear. The October 2021 report credited recognising that a repeat was not needed.
Burning 1.10 g of ethanol (M = 46.0) heats 150 g of water by 22.0 °C. The data-book value is -1367 kJ mol^-1. Calculate the percentage error in the experimental ΔcH.
Show answer
58 %. q = 150 x 4.18 x 22.0 = 13794 J; n = 0.02391 mol; ΔcH = -577 kJ mol^-1; % error = 58%.
A solid product is weighed before it is completely dry. How does this affect the percentage yield?
- It is too low
- It is unchanged
- It is too high
- It cannot be calculated
Show answer
It is too high. The water adds mass.
Suggest how to reduce the percentage uncertainty in a titre of about 10 cm³, and explain why it works.
Show answer
Use a more dilute solution in the burette (or a larger volume of the sample) so the titre is larger; the same ±0.10 cm³ is then a smaller percentage.. The absolute uncertainty is fixed by the burette.

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Last updated 8 November 2026. Spotted a mistake? Tell us.