Sodium dihydrogen phosphate, (NaH_2PO_4 )

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

Problem

Sodium dihydrogen phosphate, NaH2PO4NaH_2PO_4, and disodium hydrogen phosphate, Na2HPO4Na_2HPO_4, are often combined to prepare a buffer solution near physiological pH. When the two salts dissolve in water, the principal acid–base equilibrium in the buffer can be represented by H2PO4−(aq)+H2O(l)⇌HPO42−(aq)+H3O+(aq).H_2PO_4^{-}(aq)+H_2O(l)\rightleftharpoons HPO_4^{2-}(aq)+H_3O^{+}(aq). Respond to the following prompts about this equilibrium. (a) Identify the Brønsted–Lowry acid and the Brønsted–Lowry base in the forward reaction. State the corresponding conjugate base and conjugate acid that form. (b) Explain how the presence of both H2PO4−H_2PO_4^{-} and HPO42−HPO_4^{2-} in significant amounts allows the solution to resist changes in pH upon addition of a small amount of strong acid. (c) Describe qualitatively what happens to the ratio [HPO42−][H2PO4−]\dfrac{[HPO_4^{2-}]}{[H_2PO_4^{-}]} when a small amount of NaOH(aq)NaOH_{(aq)} is added to the buffer. Explain your reasoning using the reaction above.

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