Edexcel A-Level Chemistry A2 B 1 2 Arithmetic and Weighted Means Questions

Practise Edexcel IAL Chemistry 7.2.2 by subtracting burette readings, selecting concordant results and reporting arithmetic means with suitable precision.

Exam points

Select concordant titres within 0.2 cm³ and exclude rough or outlying titrations before averaging.
Complete burette-reading tables by subtracting initial from final volumes before calculating a mean titre.
Calculate arithmetic means from all valid results and report titre or volume values with suitable precision.

[Maximum number: 1]

Brass is an alloy of copper. The percentage by mass of copper in a sample of brass can be determined by a three-stage process.

Stage 1 Oxidation of the copper to copper(II) ions with excess concentrated nitric acid.
Stage 2 Reduction of the copper(II) ions to copper(I) ions with excess iodide ions.
Stage 3 Titration of the iodine produced in Stage 2 against a standard solution of sodium thiosulfate.

The procedure in Stage 3 is shown:
- the iodine produced in each flask from Stage 2 is titrated with $0.095 \mathrm{~mol} \mathrm{dm}^{-3}$ sodium thiosulfate solution until the iodine colour is pale yellow
- a few drops of starch indicator are added
- sodium thiosulfate is added drop by drop with swirling until the end-point is reached
- the titration is repeated until two concordant titres are obtained.

The results of the titration are shown.

Table for Question Edexcel A-Level Chemistry A2 B 1 2 Arithmetic and Weighted Means Questions question 1 — Edexcel A-Level Chemistry A2

Complete the table and calculate the mean titre of the concordant results.

calculation of titre and mean of concordant results

Example of calculation
(27.05; 26.65) 26.45 $(26.65+26.45) \div 2=26.55\left(\mathrm{~cm}^{3}\right)$

All question bank results loaded

Edexcel A-Level Chemistry A2 B 1 2 Arithmetic and Weighted Means Questions question 1