A student adds a few drops of 6 M NaOH to a well-stirred solution that is 0.20 M in…

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

Problem

A student adds a few drops of 6 M NaOH to a well-stirred solution that is 0.20 M in both HF and NaF. Which of the following best explains why the pH of the solution changes only slightly? (a) OH⁻ ions shift the

\mathrm{HF \rightleftharpoons H^{+} + F^{-}}

equilibrium to the right, producing additional

\mathrm{H^{+}}

that offsets the added base and keeps the pH nearly constant. (b) OH⁻ ions are neutralized by the tiny

[\mathrm{H^{+}}]

produced by water’s auto-ionization, so the solution’s pH changes very little. (c) OH⁻ ions react predominantly with HF, shifting the

\mathrm{HF \rightleftharpoons H^{+} + F^{-}}

equilibrium to the left and converting the added base to

\mathrm{F^{-}}

and

\mathrm{H_{2}O}

. (d) The amount of OH⁻ added is extremely small compared with the

0.20\,\text{M}

HF/NaF present, so its concentration is too low to affect the pH appreciably.

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