Question

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

Short answer

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 ,

$$\begin{gathered} \left[{\mathrm{H}}^{+}\right]=\sqrt{\mathrm{Ka}\times \mathrm{C}} \\ \left[{\mathrm{H}}^{+}\right]=\sqrt{6.2\times {10}^{-10}\times 5\times {10}^{-4}}=5.57\times {10}^{-7} \end{gathered}$$

Calculation of pH for HCN

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