Chemistry - Ionic Equilibrium Question with Solution | TestHub

Pyridinium hydrochloride is a salt of weak base and strong acid. $\begin{array}{rl}& \mathrm{pH}=-\log \left[{\mathrm{H}}^{+}\right]=-\log \left(3.6\times 10^{-4}\right)=3.44\\ & \text{ Now, }\mathrm{pH}=-\frac{1}{2}\left[\log K_w-\log K_a+\log C\right)\\ & \Rightarrow 3.44=-\frac{1}{2}\left[-14-\log K_a+\log \left(2\times 10^{-2}\right)\right]\\ & \Rightarrow 6.88=14+\log K_a+1.70\\ & \Rightarrow \log K_a=-8.82\\ & \Rightarrow K_a=\mathrm{antilog}(-8.82)=1.5\times 10^{-9}\end{array}$