A visual graph plots the log of molar solubility () of a weak acid salt (like ) on the y-axis agains — Ionic Equilibrium Chemistry Question
Question
A visual graph plots the log of molar solubility ($\log S$) of a weak acid salt (like $AgCN$) on the y-axis against the pH of the solution on the x-axis. As the pH drops strictly below the $pK_a$ of $HCN$, what visual trend does the curve exhibit?
💡 Solution & Explanation
For a salt of a weak acid, lower pH implies a higher $[H^+]$ concentration. The excess $H^+$ consumes the weak base anion ($CN^-$) to form the weak acid ($HCN$), dragging the dissolution equilibrium heavily forward. Thus, as the pH visually drops below the $pK_a$ (into the highly acidic region), the solubility $S$ increases dramatically, causing the $\log S$ vs pH graph to visually display a sharp, linear upward slope.