36 exam-style questions on Concluding and evaluating — topic I3 of Skills in the study of chemistry in the IB Chemistry syllabus — with answers and explanations. Samples below; the full set is free with an account.
In a calorimetry experiment energy is lost to the surroundings. Which change would most reduce this systematic error?
Answer: Insulate the calorimeter and add a lid
Insulation and a lid directly reduce the heat loss causing the systematic error. Repeating reduces random error only, and a finer thermometer improves precision but does not stop heat escaping.
An experimental value lies 25% below the accepted value, in the same direction across every repeat. This pattern most strongly indicates ___.
Answer: a systematic error
A consistent, one-directional deviation is the signature of a systematic error, such as calibration bias or heat loss. Random errors would scatter results either side of the mean rather than shift them all the same way.
A student measures the melting point of a product as 118–121 °C, but the pure compound melts sharply at 122 °C. What does the observed range suggest?
Answer: The product contains some impurity
An impurity lowers and broadens the melting point, giving a range that starts below the true value, exactly as seen here. A pure sample would melt sharply at 122 °C over a degree or so.
Before insulating a calorimeter a student's result had a 20% error; after insulating it had a 5% error. What does this comparison show?
Answer: Insulation improved the accuracy, quartering the error
Comparing the percentage error before and after quantifies the improvement: a fall from 20% to 5% is a fourfold reduction, bringing the result closer to the true value and so improving accuracy.
A student reads every burette meniscus with the eye above the level of the meniscus. Identify the type of error introduced and its effect on the results.
Answer: A systematic error, shifting every titre in the same direction by a similar amount
A consistent viewing angle produces a parallax error of the same sign every time, which is the definition of a systematic error, so it survives averaging untouched. It also does not show up as poor concordance: the titres agree beautifully with one another and are all wrong together.
In a combustion calorimetry experiment some of the volatile fuel evaporates from the burner between weighings. Identify the improvement that addresses this specific error.
Answer: Keeping the burner capped except while it is actually alight
Evaporation makes the recorded mass loss larger than the mass actually burned, so the enthalpy per mole comes out too small; capping the burner removes that specific loss. A lid and shield address a different error, heat loss to the surroundings, and repeats address neither because both are systematic.
That was 6 of 36 Concluding and evaluating questions.
Practise all 36 freeStateAO1 · 1–2 marks
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OutlineAO2 · 1–4 marks
A brief account or summary: the main points in two or three sentences, no evaluation.
36 Concluding and evaluating questions — free with an account. Spaced repetition, streaks and full exam simulations included.
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