Question

Calculate the pH of 5.0 × 10^-4 M HCN.

Short answer

pH of the given solution comes out to be 6.25.

Step-by-step solution

Calculation of [H+] for HCN

For weak acids like HCN ,

${\text{[H}}^{\text{+}}\text{]}=\sqrt{{\text{K}}_{\text{a}}\times \text{C}}$

${\text{[H}}^{\text{+}}\text{]}=\sqrt{6.2\times {10}^{-10}\times 5\times {10}^{-4}}=5.57\times {10}^{-7}$

Calculation of pH for HCN

${\text{pH=-log[H}}^{\text{+}}]=-\log (5.57\times {10}^{-7})=6.25$