Acid–Base Equilibria (5A) Practice Test

A blood-gas analyzer is calibrated using a bicarbonate buffer where $pK_a = 6.1$ for the $\mathrm{H_2CO_3}/\mathrm{HCO_3^-}$ pair. Two calibration solutions are prepared at 37°C:

Solution 1: $\mathrm{HCO_3^-} = 20\ \mathrm{mM}$, $\mathrm{CO_2(aq)} = 1.0\ \mathrm{mM}$

Solution 2: $\mathrm{HCO_3^-} = 10\ \mathrm{mM}$, $\mathrm{CO_2(aq)} = 1.0\ \mathrm{mM}$

Assume $\mathrm{H_2CO_3}$ tracks with $\mathrm{CO_2(aq)}$ and the Henderson–Hasselbalch relationship applies qualitatively.

Which statement best describes the system’s response when moving from Solution 1 to Solution 2?

A clinician models blood buffering by preparing 1.0 L of a weak acid buffer containing 20 mM lactic acid (HLac) and 20 mM lactate (Lac$^-$) at 37°C. For $\mathrm{HLac} \rightleftharpoons \mathrm{H^+} + \mathrm{Lac^-}$, $pK_a = 3.9$. A small bolus of strong acid is added, increasing $\mathrm{H^+}$ transiently, and the mixture is allowed to re-equilibrate without changing volume.

Based on the buffer principle, which conclusion about the buffer system is most consistent?