Edexcel IAL Chemistry 14.21 Using titration curves for buffers and Ka

Locate the flat buffer region or half-neutralisation point on the graph, then use pH = pKa or [H+] = Ka to calculate the required acid dissociation constant.

Exam points

Use half-neutralisation pH from a titration curve to calculate Ka or pKa.
Identify the buffer region where a large volume of acid or base causes little pH change.

[Maximum number: 2]

The number of moles of propanoic acid in a solution may be determined by titration with aqueous sodium hydroxide.

Calculate the pH at the point in the titration where half the acid has been neutralised. You must show your working.

An explanation that makes reference to the following points:

- (when half the acid has been neutralised)

(1)
$\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]=\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COO}^{-}\right]$

(1)

- evaluation of pH

Allow in words Allow pH at half neutralisation $=\mathrm{p} K_{\mathrm{a}}$ propanoic acid Allow $\left[\mathrm{H}^{+}\right]$ at half neutralisation =K a propanoic acid
$\mathrm{pH}=-\log _{10} 1.30 \times 10^{-5}$
=4.8861 = 4.89/4.9

Ignore SF except 1SF

Correct answer scores 2

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14.21—How to use a weak acid-strong base or strong acid-weak base titration curve to: i demonstrate buffer action ii determine Ka question 1