$\begin{aligned}
& \mathrm{HX} \rightleftharpoons \mathrm{H}^{+}+\mathrm{X}^{-} \quad \mathrm{K}{\mathrm{a}}=1.2 \times 10^{-5} \
& 0.03 \mathrm{M} \
& 0.03-\mathrm{x} \quad \mathrm{x} \quad \mathrm{x} \
& \mathrm{K}{\mathrm{a}}=1.2 \times 10^{-5}=\frac{\mathrm{x}^2}{0.03-\mathrm{x}} \
& 0.03-\mathrm{x} \approx 0.03\left(\mathrm{~K}_{\mathrm{a}} \text { is very small }\right) \
& \frac{\mathrm{x}^2}{0.03}=1.2 \times 10^{-5} \
& \mathrm{x}=6 \times 10^{-4}
\end{aligned}$
Final solution : 0.03−x+x+x $\begin{aligned}
& =0.03+x=0.03+6 \times 10^{-4} \
& \Pi=\left(0.03+\left(6 \times 10^{-4}\right)\right) \times 0.083 \times 300 \
& =76.19 \times 10^{-2} \approx 76 \times 10^{-2}
\end{aligned}$