A sample of a solution that is in and in is titrated with

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

100\,\text{mL}

sample of a solution that is

0.20\,\text{M}

\mathrm{NH_3}

and

0.20\,\text{M}

\mathrm{NH_4Cl}

is titrated with

0.40\,\text{M}

\mathrm{HCl}

. The pH stays nearly constant as up to

50\,\text{mL}

\mathrm{HCl}

is added, but after about

60\,\text{mL}

\mathrm{HCl}

, the pH drops sharply. Which of the following best accounts for the sudden pH decrease beyond

50\,\text{mL}

of added

\mathrm{HCl}

? (a) After

50\,\text{mL}

, the added acid has doubled the total volume, diluting the buffer components so much that the pH falls sharply even without further reaction. (b) At

50\,\text{mL}

, virtually all

\mathrm{NH_3}

has been converted to

\mathrm{NH_4^+}

, so additional

\mathrm{HCl}

adds free

\mathrm{H^+}

to the solution, causing a steep pH decline. (c) Beyond

50\,\text{mL}

, all

\mathrm{NH_4^+}

has been consumed, leaving only

\mathrm{NH_3}

; without the conjugate acid present, the buffer cannot neutralise more acid, so the pH plummets. (d) Excess

\mathrm{Cl^-}

ions after

50\,\text{mL}

precipitate solid

\mathrm{NH_4Cl}

, removing the buffer pair from solution and allowing the pH to fall abruptly.