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CAIE A-Level Chemistry P3.8 Errors, Uncertainty and Improvements

Practise identifying significant errors, quantifying uncertainty and proposing realistic experimental improvements.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • distinguish random variation from a systematic shift and identify the most significant source in the method
  • calculate or state actual and percentage uncertainty from the measurement and apparatus resolution
  • propose a specific modification that addresses the named error, then state how it improves the evidence

P3.8—Errors, uncertainty, improvements and extensions question 1

[Maximum number: 1]

The reaction between Na2 S2O3(aq)\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq}) and HCl(aq) is monitored at constant temperature.

Na2 S2O3+2HCl→2NaCl+SO2+S+H2O\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}+2 \mathrm{HCl} \rightarrow 2 \mathrm{NaCl}+\mathrm{SO}_{2}+\mathrm{S}+\mathrm{H}_{2} \mathrm{O}

Fig. 4.1 shows how the concentration of HCl(aq) varies with time.

Fig. 4.1

Fig. 4.1

Explain why the rate of reaction cannot be monitored accurately by measuring the volume of SO2( g)\mathrm{SO}_{2}(\mathrm{~g}) produced in this reaction.

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