IB Chemistry HL 3.1.17 Buffer pH and composition Question Bank

Practise IB Chemistry HL 3.1.17 by applying buffer ph and composition to exam-style questions.

Exam points

identify the relevant chemical structure, process or evidence
apply the correct chemical model or equation
evaluate the result using chemical reasoning and data

[Maximum number: 3]

Nitrous acid, $\mathrm{HNO}_{2}$ , is a weak acid which can be used to make acidic buffers.

A $1.00 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of nitrous acid was prepared. This solution contains two conjugate acid-base pairs.

This $1.00 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of nitrous acid was used to prepare a buffer with pH 3.00 .

Calculate the concentration of the conjugate base of nitrous acid required to make this buffer. The $\mathrm{pK}_{\mathrm{a}}$ of nitrous acid is 3.25.

Concentration of conjugate base:

（a）

（ii）

$K_{a}<10^{-3.25}>=5.62 \times 10^{-4}$

« $\left[\mathrm{H}^{+}\right]=10^{-3.00}$ » =0.001 «mol dm ${ }^{-3}$ »
$\text { « }\left[\mathrm{A}^{-}\right]=\left(5.62 \times 10^{-4} \times 1.00\right) / 0.001 »=0.562 \text { «mol } \mathrm{dm}^{-3} \text { » }$

Alternative solution：
$\mathrm{pH}=\mathrm{p} K_{a}+\log _{10}[\text { salt }] /[\text { acid }]$ $\text { « } \log _{10}[\text { salt }] /[\text { acid }] \text { 》 }=-0.25$

OR
«[salt]/[acid]» = 0.562、

《 $\left[\mathrm{A}^{-}\right]=0.562 / 1.00 »=0.562$ «mol $\mathrm{dm}^{-3}$ » ↓

Award［3］for correct final answer．

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3.1.17 (HL)—Buffer pH and composition question 1