Edexcel A-Level Chemistry Assessed Mathematical Skills

Practise Edexcel A-Level Chemistry AS and A2 mathematical skills by using titres, graphs, percentages, units and measurement conventions.

Covered syllabus points

B.0 Arithmetic and numerical computation B.1 Handling data B.2 Algebra B.3 Graphs B.4 Geometry and trigonometry

[Maximum number: 5]

This question is about coloured transition metal complex ions.

(a)

（c）Colorimetry was used to determine the concentration of a solution of copper（II）sulfate．
Six solutions of copper（II）sulfate of known concentration were prepared and their absorbance at a wavelength of 635 nm measured．The results are shown．

Table for Question (a) — Edexcel A-Level Chemistry A2

[ 5 ]

(i)

（i）Plot a graph of absorbance against concentration using the data in the table．
（3）

Table for Question (i) — Edexcel A-Level Chemistry A2

[ 3 ]

(ii)

$50.0 \mathrm{~cm}^{3}$ of a solution of copper(II) sulfate of unknown concentration was pipetted into a $250.0 \mathrm{~cm}^{3}$ volumetric flask and the volume made up to the

mark with deionised water.
The absorbance at a wavelength of 635 nm for this diluted copper(II) sulfate solution was 0.72.

Determine the concentration of the original copper(II) sulfate solution. You must show your working on the graph.

[ 2 ]

use of graph to determine
concentration from 0.72 absorbance

(1)

Example of graph

Allow answers in the range from $0.255-0.265\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)$

Do not award M1 if working absent from graph

Scaling up from diluted concentration
$=0.26 \times(250 \div 50)=1.3\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)$

Allow answers in the range $1.275-1.325\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)$ from 0.255-0.265

Ignore SF except 1SF

TE from incorrect graphical reading

[Maximum number: 4]

A student investigated the kinetics of the reaction between bromide ions, $\mathrm{Br}^{-}$ , and bromate(V) ions, $\mathrm{BrO}_{3}^{-}$ , in aqueous acid.

5 \mathrm{Br}^{-}(\mathrm{aq})+\mathrm{BrO}_{3}^{-}(\mathrm{aq})+6 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow 3 \mathrm{Br}_{2}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

To determine the rate equation for the reaction, the student varied the concentration of bromide ions, bromate(V) ions and acid in turn.

The effect of the concentration of bromide ions was investigated first.

Procedure

Step 1 Add $10.0 \mathrm{~cm}^{3}$ of $0.0050 \mathrm{~mol} \mathrm{dm}^{-3}$ potassium bromate(V), $15.0 \mathrm{~cm}^{3}$ of acidified methyl orange indicator solution and $5.0 \mathrm{~cm}^{3}$ of $0.00010 \mathrm{~mol} \mathrm{dm}^{-3}$ aqueous phenol to a beaker labelled P.

Step 2 Prepare the contents of beaker Q for Run 1 as specified in the table.
Step 3 Pour the contents of beaker Q into beaker P and start a timer. Pour the contents of beaker P back into beaker Q and place beaker Q on a white tile.

Step 4 Stop the timer as soon as the mixture turns colourless. Record the time, along with the temperature of the solution.

Step 5 Repeat Steps 1 to 4 for the remaining runs.

Table for Question Assessed mathematical skills and measurement conventions question 2 — Edexcel A-Level Chemistry A2

(a)

The student's results are shown.

[ 4 ]

(i)

Complete the table by calculating the remaining values of 1 / t.

[ 1 ]

table complete with both 1 / t values to 2 SF

Example of completed table:

Volume of $\mathrm{KBr} / \mathrm{cm}^{3}$ & 10.0 & 8.0 & 6.0 & 5.0 & 4.0 & 3.0

Time,t / s & 23 & 24 & 32 & 39 & 48 & 64

$(1 / t) / \mathrm{s}^{-1}$ & 0.043 & 0.042 & 0.031 & 0.026 & $\underline{\mathbf{0. 0 2 1}}$ & $\underline{\mathbf{0. 0 1 6}}$

Temperature $/{ }^{\circ} \mathrm{C}$ & 18 & 22 & 22 & 22 & 22 & 22

(ii)

Plot a graph of 1 / t against the volume of KBr. Include a line of best fit.
(3)

Figure for Question (ii) — Edexcel A-Level Chemistry A2

[ 3 ]

axes labelled correctly with units and suitable scale (1)

- all points plotted correctly (1)

- straight line of best fit passing through the origin and missing anomalous point at $10 \mathrm{~cm}^{3} \mathrm{KBr} \quad$ (1)

Example of graph:

Do not award M1 if variables plotted the wrong way round

Points plotted must cover at least 50% of the graph in both directions

Allow error margin of ± one small square TE on 1 / t values from (c)(i)

Allow BFL which would pass through the origin if extrapolated

Do not award M3 if axes do not start at 0,0

TE on points plotted provided line goes through the origin

[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 Assessed mathematical skills and measurement conventions question 3 — 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: 3]

This question is about the reaction between nitrogen monoxide and oxygen.

\mathrm{NO}(\mathrm{~g})+1 / 2 \mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{NO}_{2}(\mathrm{~g})

(a)

The results of a series of kinetics experiments are shown.

The data for experiments 1-5 were plotted on a graph.

Figure for Question (a) — Edexcel A-Level Chemistry A2

[ 3 ]

(i)

Draw a best-fit line on the graph.

[ 1 ]

(ii)

Calculate the rate constant for this reaction using the data from experiment 1 and your rate equation. Include units in your answer.

[ 2 ]

substitution or rearrangement

- answer and units

Example of a calculation:
$\begin{aligned} 0.040=\mathrm{k}(0.010)^{2}(0.05) \mathrm{k}=\frac{0.040}{(0.010)^{2}(0.05)} \end{aligned}$ $=8000 \mathrm{dm}^{6} \mathrm{~mol}^{-2} \mathrm{~s}^{-1}$
Allow units in any order
Allow $\mathrm{s}^{-}$
Allow $\mathrm{dm}^{6} / \mathrm{mol}^{2} \mathrm{~s}$ but not $\mathrm{dm}^{6} / \mathrm{mol}^{2} / \mathrm{s}$
Allow TE on a(iv) for both marks, units must match
order for M2
Correct answer with units scores 2, even if expression
in Q18(a)(iv) is incorrect

(2)

All question bank results loaded

Covered syllabus points

Assessed mathematical skills and measurement conventions question 1

Question (a)

Question (i)

Question (ii)

Assessed mathematical skills and measurement conventions question 2

Question (a)

Question (i)

Question (ii)

Assessed mathematical skills and measurement conventions question 3

Assessed mathematical skills and measurement conventions question 4

Question (a)

Question (i)

Question (ii)