Question

Addition of solid potassium cyanide to pure water causes:

• A. an increase in pH
• B. decrease pH
• C. no change in pH
• D. no change in electrical conductivity

Answer 1

The correct option is B decrease pH

B. Decrease in pH
The KCN(s) will dissociate into K+(aq) and CN_(aq) by the following reaction:
KCN(s)$\to$ K${}^{+}$(aq)+CN${}^{-}$(aq)
K+ will not hydrolyze, but the CN- anion will attract an H+away from the water:
CN${}^{-}$(aq)+H${}_2$O(l)$\rightleftharpoons$HCN(aq)+OH${}^{-}$(aq)
Because this reaction produces OH${}^{-}$, the resulting solution will be basic and cause a pH>7.
KCN can be thought of the salt made by combining the strong base KOH with the weak acid HCN: K${}^{+}$ + OH${}^{–}$ $\to$ KOH. When solid KCN dissolves in water, this process is reversed, yielding a solution of the two ions. K${}^{+}$, being a "strong ion", does not react with water. But the "weak ion" CN${}^{–}$, being the conjugate base of a weak acid, will have a tendency to abstract protons from water: CN${}^{–}$ + H${}_{2}O$ $\to$ HCN + OH${}^{-}$, causing the solution to remain slightly alkaline.