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.