Question

A solution of a weak acid, HA, has a concentration of 0.100 M. What is the concentration of hydronium ion and the pH of this solution if the $K_a$ value for this acid is $1.0\times {10}^{-5}$?

• A. $1.0\times {10}^{-3}$ and pH = 11
• B. $1.0\times {10}^{-6}$ and pH = 6
• C. $1.0\times {10}^{-4}$ and pH = 8
• D. $3.0\times {10}^{-4}$ and pH = 4
• E. $1.0\times {10}^{-3}$ and pH = 3

Answer 1

The correct option is E $1.0\times {10}^{-3}$ and pH = 3

$\left[H^{+}\right]=\sqrt{C{K}_a}=\sqrt{0.100\times 1.0\times {10}^{-5}}=1.0\times {10}^{-3}M$
$pH=-log\left[H^{+}\right]=-log1.0\times {10}^{-3}=3$