A dimer, X, is stable when solid but a dynamic equilibrium is set up in solution.
X(aq) ⇌ 2Y(aq)
A solution of X has an initial concentration of 0.50 mol dm⁻³. When equilibrium has been reached [X(aq)] has fallen to 0.25 mol dm⁻³.
The changes in [X(aq)] and [Y(aq)] are plotted against time until equilibrium is reached. The value of K_c is then calculated.
Which graph and value for K_c are correct?

Q: What topic does this question test?

This question tests Equilibria in A-Level Chemistry (syllabus code 9701). It is worth 1 mark.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Chemistry Oct/Nov 2010 examination, Paper 1 Variant 2 (9701_w10_qp_12.pdf).

Question

A dimer, X, is stable when solid but a dynamic equilibrium is set up in solution.
X(aq) ⇌ 2Y(aq)
A solution of X has an initial concentration of 0.50 mol dm⁻³. When equilibrium has been reached [X(aq)] has fallen to 0.25 mol dm⁻³.
The changes in [X(aq)] and [Y(aq)] are plotted against time until equilibrium is reached. The value of K_c is then calculated.
Which graph and value for K_c are correct?

Oracle Prep · Accepted Answer

The correct answer is C: [Graph C showing [X] decreasing from 0.50 to 0.25 and [Y] increasing from 0 to 0.50, with Kc = 1].

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