Phosphoric acid,

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

Phosphoric acid,

\mathrm{H_{3}PO_{4}}

, is a triprotic acid with the following first two acid-dissociation constants at

25\,^{\circ}\text{C}

\mathrm{H_{3}PO_{4}} \rightleftharpoons \mathrm{H^{+}} + \mathrm{H_{2}PO_{4}^{-}} \quad K_{a_1} = 7.1 \times 10^{-3}

\mathrm{H_{2}PO_{4}^{-}} \rightleftharpoons \mathrm{H^{+}} + \mathrm{HPO_{4}^{2-}} \quad K_{a_2} = 6.3 \times 10^{-8}

A student prepares a solution that is

0.20\,\text{M}

\mathrm{NaH_{2}PO_{4}}

and

0.20\,\text{M}

\mathrm{Na_{2}HPO_{4}}

. (a) Write the net ionic equation for the equilibrium that is primarily responsible for controlling the pH of the prepared solution. (b) Explain why the mixture described functions as an effective buffer. Your explanation must reference both the composition of the solution and the numerical values of

K_{a_1}

and

K_{a_2}

. (c) The student proposes an alternative mixture that is equimolar

\mathrm{H_{3}PO_{4}}

and

\mathrm{NaH_{2}PO_{4}}

. Predict whether this second mixture would provide comparable buffering capacity near the same pH as the original mixture. Justify your prediction.

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