Carbonic acid,

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

Problem

Carbonic acid,

\mathrm{H_{2}CO_{3}}

, is a diprotic acid whose first acid dissociation constant at

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

K_{a1}=4.3 \times 10^{-7}

. A chemist mixes

50.0\,\text{mL}

0.100\,\text{M}

\mathrm{H_{2}CO_{3}}

with

25.0\,\text{mL}

0.100\,\text{M}

NaOH at

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

. Assume that only the first ionization of carbonic acid significantly affects the pH and that volumes are additive. (a) Write the balanced chemical equation describing the reaction that occurs when the two solutions are mixed. (b) Calculate the number of moles of each relevant species present after the reaction has gone to completion. (c) Determine the pH of the reaction mixture immediately after mixing. (d) The mixture is then diluted with distilled water to a final volume of

250\,\text{mL}

. Calculate the pH after dilution and explain, with reasoning, why the pH changes (or does not change) upon dilution.

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