Edexcel A-Level Chemistry Graph Skills

Practise Edexcel A-Level Chemistry graph skills through plotted kinetics and calibration graphs, best-fit lines, gradient work and graphical readings.

Exam points

plot chemistry data with labelled axes, units, suitable scales and accurate points
draw best-fit lines or curves, including origin lines and anomalous-point decisions
read concentrations, gradients or unknown masses from calibration and kinetics graphs

Covered syllabus points

B.3.1—Translating between data forms B.3.2—Plotting two variables B.3.3—Slope and intercept of a linear graph B.3.4—Rate from a linear graph B.3.5—Tangents and instantaneous rates

[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: 3]

The equation for the formation of ammonia in the Haber Process is shown

1 / 2 \mathrm{~N}_{2}(\mathrm{~g})+1 \frac{1}{2} \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{NH}_{3}(\mathrm{~g})

(a)

The value of the total entropy change, $\Delta S_{\text {total }}$ , varies with temperature.

Data for the value of $\Delta S_{\text {total }}$ at different temperatures but at standard pressure of 100 kPa are given for this reaction.

Plot a graph of $\Delta S_{\text {total }}$ against 1 / T on the grid.
Include a line of best fit.

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

[ 2 ]

(b)

The relationship between $\Delta S_{\text {total }}$ and 1 / T can be found by combining the two equations:

\Delta S_{\text {total }}=\Delta S_{\text {surroundings }}+\Delta S_{\text {system }}

and $\Delta S_{\text {surroundings }}=-\Delta H / T$
to give

\Delta S_{\text {total }}=-\Delta H / T+\Delta S_{\text {system }}

[ 1 ]

(i)

Determine the gradient of the line plotted in (b), including units in your answer.

[ 1 ]

An answer that makes reference to the following points:

- uses the line or points from the data to calculate the gradient and units

Example of calculation
$\text { Gradient }=\frac{8.27 \times 10^{-2}--0.76 \times 10^{-2}}{4.00 \times 10^{-3}-2.00 \times 10^{-3}}$ $=45.15 \mathrm{~kJ} \mathrm{~mol}^{-1}$
Allow an answer between 42.0-48.0
Ignore SF except 1 SF

All question bank results loaded

Covered syllabus points

B.3 Graphs question 1

Question (a)

Question (i)

Question (ii)

B.3 Graphs question 2

Question (a)

Question (b)

Question (i)