Question

An aqueous solution of HCI 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

The correct option is D decreases thousand-fold

$pH=2\Rightarrow [H^{+}{]}_i={10}^{-2}M$
$pH=5\Rightarrow [H^{+}{]}_t={10}^{-5}M$
$\frac{[H^{+}{]}_f}{[H^{+}{]}_i}=\frac{{10}^{-5}}{{10}^{-2}}=\left(\frac{1}{1000}\right)$
So, $H^{+}$ concentration decreases thousand fold.