Edexcel A-Level Chemistry Handling Data

Practise Edexcel A-Level Chemistry handling-data skills through concordant titre selection, mean calculations, uncertainty ranges and significant-figure decisions.

Exam points

select concordant titres from burette tables before calculating a valid mean titre
calculate percentage uncertainty from repeated readings and measured changes
complete data tables with reciprocal or equilibrium values to stated significant figures

Covered syllabus points

B.1.1—Significant figures B.1.2—Arithmetic and weighted means B.1.3—Measurement uncertainty

[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 Handling data 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 $(26.65+26.45) \div 2=26.55\left(\mathrm{~cm}^{3}\right)$

[Maximum number: 1]

In a redox titration, $0.0500 \mathrm{~mol} \mathrm{dm}^{-3}$ potassium manganate(VII) is titrated against $25.0 \mathrm{~cm}^{3}$ of a solution containing iron(II) ions.

\mathrm{MnO}_{4}^{-}+8 \mathrm{H}^{+}+5 \mathrm{Fe}^{2+} \rightarrow 5 \mathrm{Fe}^{3+}+\mathrm{Mn}^{2+}+4 \mathrm{H}_{2} \mathrm{O}

The mean titre of potassium manganate(VII) is $18.95 \mathrm{~cm}^{3}$ .

What is the percentage error in a single titre of $18.95 \mathrm{~cm}^{3}$ ?
[The uncertainty in a single burette reading is $\pm 0.05 \mathrm{~cm}^{3}$ ]

$\pm 0.20 \%$

$\pm 0.26 \%$

$\pm 0.40 \%$

$\pm 0.53 \%$

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Covered syllabus points

B.1 Handling data question 1

B.1 Handling data question 2