Edexcel IAL Chemistry 3.P10 Accuracy and precision

Compare measured values or their range with the accepted value, inspect scatter around the trend and decide whether a single anomaly changes the conclusion.

Exam points

Distinguish uncertainty range from accuracy relative to an accepted or true value.
Use a best-fit trend and anomalous point to decide whether repetition is necessary.

[Maximum number: 2]

In an experiment to measure the enthalpy change of combustion of ethanol, a student uses the apparatus shown.

Figure for Question 3.P10—Accuracy and precision question 1 — Edexcel A-Level Chemistry AS

The student repeated the experiment using the same apparatus, in an attempt to improve its accuracy. The water was heated for 15 minutes before measuring the final temperature and reweighing the burner.

The data book value for the standard enthalpy change of combustion of ethanol is $-1367 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The teacher said that the changes would improve the percentage uncertainty in the measurement of mass and temperature but have little effect on the accuracy of the value obtained.

Explain how the difference between uncertainty and accuracy led the teacher to make this statement.

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3.P10—Accuracy and precision question 1