Define the term buffer solution.

Explain, using appropriate equations, how this solution acts as a buffer solution.

Calculate the concentration, in \({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\), of sodium propanoate in this buffer solution.

The \({\text{p}}{K_{\text{a}}}\) of propanoic acid is 4.87 at 298 K.

a solution that resists changes in pH / changes pH slightly / OWTTE;

when small amounts of an acid/\({{\text{H}}^ + }\) or a base/alkali/\({\text{O}}{{\text{H}}^ - }\) are added;

addition of acid:

\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{CO}}{{\text{O}}^ - }{\text{(aq)}} + {{\text{H}}^ + }{\text{(aq)}} \to {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH(aq)}}\) / propanoate ions combine with \({{\text{H}}^ + }\) ions to form undissociated propanoic acid;

addition of base:

\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH(aq)}} + {\text{O}}{{\text{H}}^ - }{\text{(aq)}} \to {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{CO}}{{\text{O}}^ - }{\text{(aq)}} + {{\text{H}}_2}{\text{O(l)}}\) / addition of \({\text{O}}{{\text{H}}^ - }\) removes \({{\text{H}}^ + }\) and more propanoic acid dissociates/ionizes;

Ignore state symbols.

Accept reversible arrows.

Award [1 max] if correct equations are given without reference to addition of acid or alkali.

\({K_{\text{a}}} = \frac{{[{{\text{H}}^ + }{\text{(aq)][C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{CO}}{{\text{O}}^ - }{\text{(aq)]}}}}{{[{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH(aq)]}}}}/{\text{pH}} = {\text{p}}{K_{\text{a}}} + \log \left( {\frac{{{\text{[base]}}}}{{{\text{[acid]}}}}} \right)\);

\({K_{\text{a}}} = 1.3 \times {10^{ - 5}}/{10^{ - 4.87}}\) and \([{{\text{H}}^ + }] = 1.3 \times {10^{ - 4}}/{10^{ - 3.87}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{) }}/\log \frac{{{\text{[C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{CO}}{{\text{O}}^ - }]}}{{[{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH]}}}} = 3.87 - 4.87 =  - 1\);

\(\left( {[{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COOH]}} = \frac{{7.41}}{{74.09}} = } \right)0.100/1.00 \times {10^{ - 1}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);

\({\text{([C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{COONa]}} = )0.010/1.0 \times {10^{ - 2}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}})\);

Award [4] for correct final answer.

Accept corresponding use of \({[{H_3}O]^ + }\) for \([{H^ + }]\), [acid] for \([C{H_3}C{H_2}COOH]\), and [base] or [salt] for \([C{H_3}C{H_2}CO{O^ - }]\) throughout.

Most candidates were able to give a definition of buffer solutions including the detail that pH does not change significantly when small amounts of acid or alkali are added. The explanation of the action of buffers proved to be more challenging with only the stronger candidates giving a complete response in terms of protonation of the conjugate base and increased dissociation of the acid. The calculation of equilibrium concentrations from \({\text{p}}{K_{\text{a}}}\) values was better done than in previous sessions, but still proved too difficult for many. The need to change the units of concentration of propanoic acid made this an additional obstacle in this demanding question.

Most candidates were able to give a definition of buffer solutions including the detail that pH does not change significantly when small amounts of acid or alkali are added. The explanation of the action of buffers proved to be more challenging with only the stronger candidates giving a complete response in terms of protonation of the conjugate base and increased dissociation of the acid. The calculation of equilibrium concentrations from \({\text{p}}{K_{\text{a}}}\) values was better done than in previous sessions, but still proved too difficult for many. The need to change the units of concentration of propanoic acid made this an additional obstacle in this demanding question.

Most candidates were able to give a definition of buffer solutions including the detail that pH does not change significantly when small amounts of acid or alkali are added. The explanation of the action of buffers proved to be more challenging with only the stronger candidates giving a complete response in terms of protonation of the conjugate base and increased dissociation of the acid. The calculation of equilibrium concentrations from \({\text{p}}{K_{\text{a}}}\) values was better done than in previous sessions, but still proved too difficult for many. The need to change the units of concentration of propanoic acid made this an additional obstacle in this demanding question.