Edexcel IAL Chemistry Arithmetic and Weighted Means

Practise Edexcel IAL Chemistry arithmetic and weighted means through titration tables, concordant titres, outlier choices and mean-volume calculations.

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 B.1.2—Arithmetic and weighted means 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 $(26.65+26.45) \div 2=26.55\left(\mathrm{~cm}^{3}\right)$

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B.1.2—Arithmetic and weighted means question 1