CAIE A-Level Chemistry 25.1.5 Buffer solution

Practise defining buffers, choosing acid-salt mixtures and writing equations that show how added H+ or OH- is removed.

Exam points

define a buffer as resisting pH change when small amounts of acid or base are added
write equations showing H+ and OH- being removed by a buffer pair

[Maximum number: 2]

(a)

Solution D is made by mixing $100 \mathrm{~cm}^{3}$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ and $100 \mathrm{~cm}^{3}$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{NaCl}$ .

The pH of solution D is measured as small amounts of $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$ are added to it, and when small amounts of NaOH(aq) are added to it.

Solution D only acts as a buffer solution when one of these solutions is added to it.

[ 2 ]

(i)

Complete the sentence and write an equation for the reaction that occurs.

Solution D acts as a buffer when is added to it. equation

[ 1 ]

NaOH AND $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{OH}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COO}^{-}+\mathrm{H}_{2} \mathrm{O}$

(ii)

Complete the sentence and explain why solution D does not act as a buffer when the other solution is added.

Solution D does not act as a buffer when is added to it. explanation

[ 1 ]

$\mathrm{H}_{2} \mathrm{SO}_{4}$ AND conjugate base of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ is not present

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25.1.5—Buffer solution question 1

Question (a)

Question (i)

Question (ii)