Edexcel A-Level Chemistry A2 B 2 3 Substitution Into Algebraic Equations Questions

Practise Edexcel IAL Chemistry 7.3.3 by substituting gradients, density and percentage data into multi-step equations and reporting correct units.

Exam points

Substitute gradients into relationships such as gradient = −H/R and report enthalpy with correct units.
Combine density, percentage by volume and molar mass steps to calculate moles in multi-stage problems.
Use previous calculated values in energy or electrode equations, carrying units through to the final answer.

[Maximum number: 1]

This question is about electrochemical cells.

The Direct Methanol Fuel Cell (DMFC) uses methanol and oxygen to generate electricity. When the cell operates, the methanol reacts with water at one electrode to produce carbon dioxide and hydrogen ions.
At the other electrode the oxygen reacts with hydrogen ions to form water.

The reduction half-cell in the DMFC has a standard electrode potential $E^{\ominus}=+1.23 \mathrm{~V}$ .
When the solution concentration changes, the electrode potential changes so that $E=+1.20 \mathrm{~V}$ .
The relationship between these values is given by the equation

E=E^{\ominus}+4.277 \times 10^{-3} \ln \mathbf{X}

where X is the concentration of the solution in $\mathrm{mol} \mathrm{dm}^{-3}$ .
Calculate the value of X in the non-standard half-cell.

calculation of concentration X

Example of calculation:
$\begin{aligned} & c(X)=\text { inverse } \ln \left(\left((1.20-1.23) \div 4.277 \times 10^{-3}\right)\right) & =8.9897 \times 10^{-4}\left(\mathrm{~mol} \mathrm{dm}^{-3}\right) \end{aligned}$

Ignore SF except 1SF

Do not award $9 \times 10^{-4}$

Ignore any units given with the numerical value

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Edexcel A-Level Chemistry A2 B 2 3 Substitution Into Algebraic Equations Questions question 1