Question

An aqueous solution of $HCl$ has a pH of $2.0$. When water is added to increase the $pH$ to $5.0$, the hydrogen ion concentration

• A. remains the same
• B. decreases three-fold
• C. increases three-fold
• D. decreases thousand-fold

Answer 1

Answer: (D)

As we know
$pH=-\log \left[H^{+}\right]$
As the initial $pH$ of aqueous solution $=2$
$\therefore$ Concentration of $H^{+},\left[H^{+}\right]=10^{-2}$
As the final $pH$ of aqueous solution $=5$
$\therefore \left[H^{+}\right]=10^{-5}$
$\therefore \frac{{\left[H^{+}\right]}_f}{{\left[H^{+}\right]}_i}=\frac{10^{-5}}{10^{-2}}$
$=10^{-3}$
Thus, $H^{+}$concentration decreases by thousand fold.