Two buffer solutions are prepared from the same weak acid
CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Two buffer solutions are prepared from the same weak acid, whose pKa is 4.76.
• Buffer X is made so that [A−]:[HA]=1:1.
• Buffer Y is made so that [A−]:[HA]=10:1.
Which statement best describes the relative pH values of the two buffers?
(a) Buffer X has the higher pH because the equal concentrations of A− and HA make the logarithmic term zero and leave pH equal to pKa, which is larger than when the ratio is 10:1.
(b) Both buffers have the same pH because the Henderson–Hasselbalch equation shows pH depends only on pKa; the [A−]/[HA] ratio only affects buffering capacity.
(c) Buffer Y has the higher pH because a larger [A−]/[HA] ratio shifts the equilibrium toward lower [H+].
(d) The pH cannot be predicted from the ratio alone because the absolute molar concentrations of A− and HA are required.