\textbf{231 -- Which statement is correct?}
\begin{enumerate}
\item Diethylammonium ion, the conjugate acid of diethylamine, is $(\text{CH}_\gamma-\text{CH}_\gamma)_\gamma\text{N}^-$.
\item The pH of a $0.5\%$ molar hydrochloric acid solution is $1.7$.
\item If the concentration of a strong acid solution is doubled, the pH decreases by one unit.
\item In a buffer solution, the pH equals $\text{p}K_a$ of the acid when the molarities of acid and salt are equal.
\end{enumerate}