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Edexcel A-Level Chemistry A2 B 1 Handling Data Questions

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

Syllabus
Pearson Edexcel International Advanced Subsidiary/Advanced Level
Course
Chemistry XCH11/YCH11
Level
A2

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 figuresB.1.2—Arithmetic and weighted meansB.1.3—Measurement uncertainty

Question 1

[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 moldm−30.095 \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 1 — Edexcel A-Level Chemistry A2

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

Question 2

[Maximum number: 1]

In a redox titration, 0.0500 moldm−30.0500 \mathrm{~mol} \mathrm{dm}^{-3} potassium manganate(VII) is titrated against 25.0 cm325.0 \mathrm{~cm}^{3} of a solution containing iron(II) ions.

MnO4−+8H++5Fe2+→5Fe3++Mn2++4H2O\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 cm318.95 \mathrm{~cm}^{3}.

What is the percentage error in a single titre of 18.95 cm318.95 \mathrm{~cm}^{3} ?
[The uncertainty in a single burette reading is ±0.05 cm3\pm 0.05 \mathrm{~cm}^{3} ]

A

±0.20%\pm 0.20 \%

B

±0.26%\pm 0.26 \%

C

±0.40%\pm 0.40 \%

D

±0.53%\pm 0.53 \%

Question 3

[Maximum number: 2]

When excess calcium hydroxide is added to water, a reversible reaction occurs resulting in this equilibrium.

Ca(OH)2( s)⇌Ca2+(aq)+2OH−(aq)\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{~s}) \rightleftharpoons \mathrm{Ca}^{2+}(\mathrm{aq})+2 \mathrm{OH}^{-}(\mathrm{aq})

The equilibrium constant, KcK_{c}, for this reaction is

Kc=[Ca2+(aq)][OH−(aq)]2K_{\mathrm{c}}=\left[\mathrm{Ca}^{2+}(\mathrm{aq})\right]\left[\mathrm{OH}^{-}(\mathrm{aq})\right]^{2}

A student carries out an experiment to determine the concentration of a saturated solution of calcium hydroxide, and the value of KcK_{c}.

Procedure

Step 1 Weigh approximately 2 g of powdered calcium hydroxide into a conical flask.
Step 2 Add about 100 cm3100 \mathrm{~cm}^{3} of deionised water to the flask. Stopper the flask and shake for about a minute. Leave the stoppered flask for one day.

Step 3 Filter the contents of the flask.
Step 4 Titrate 25.0 cm325.0 \mathrm{~cm}^{3} portions of the filtrate with 0.100 moldm−30.100 \mathrm{~mol} \mathrm{dm}^{-3} hydrochloric acid in the presence of a suitable indicator.

Results from the titrations are shown.

Table for Question 3 — Edexcel A-Level Chemistry A2

Complete the table and calculate the mean titre in cm3\mathrm{cm}^{3}.

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