Consider the fractional composition diagram for a weak diprotic acid plotted against pH. The visual — Ionic Equilibrium Chemistry Question
Question
Consider the fractional composition diagram for a weak diprotic acid $H_2A$ plotted against pH. The visual graph features three bell-like or sigmoidal curves: Curve A starts at fraction 1.0 at pH 0, Curve B peaks at an intermediate pH, and Curve C approaches fraction 1.0 at high pH. Which chemical species does Curve B visually represent?
💡 Solution & Explanation
In a fractional composition graph for a diprotic acid, the fully protonated species ($H_2A$) dominates at very low pH (Curve A). As pH increases, the first proton is lost, and the intermediate amphiprotic species ($HA^-$) reaches its maximum fractional concentration between $pK_{a1}$ and $pK_{a2}$ (Curve B). At highly basic pH values, the fully deprotonated species ($A^{2-}$) strictly dominates (Curve C). Therefore, Curve B visually represents the $HA^-$ species.