The acid dissociation constant Ka of HCN is 5 * 10⁻¹⁰ at 25 degrees C. To maintain a buffer solution at a constant pH of 9, what volume of 5 M KCN solution must be added to 10 mL of 2 M HCN solution?

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4 mL. With pKa ~ 9.30 and target pH = 9, the Henderson-Hasselbalch equation gives [CN-]/[HCN] = 10^(9-9.30) = 10^(-0.30) ~ 0.5. Setting up mole ratios with moles of HCN = 0.02 mol and letting V be the volume of KCN added gives the required volume.

The acid dissociation constant Ka of HCN is 5 * 10⁻¹⁰ at 25 degrees C. To maintain a buffer solution at a constant pH of 9, what volume of 5 M KCN solution must be added to 10 mL of 2 M HCN solution?

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