Boric acid, is used as a mild antiseptic. What is the pH of a aqueous solution of boric acid? The hy — Ionic Equilibrium Chemistry Question
Question
Boric acid, $\text{B(OH)}_3$ is used as a mild antiseptic. What is the pH of a $0.0025\text{ M}$ aqueous solution of boric acid? The hydrogen ion arises principally from the reaction:<br><br>$\text{B(OH)}_3\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{B(OH)}_4^-\text{(aq)} + \text{H}^+\text{(aq)}; K = 4.0 \times 10^{-10}$
Answer: 6
💡 Solution & Explanation
Given $C = 0.0025\text{ M}$ and $K_a = 4.0 \times 10^{-10}$. Using the weak acid approximation, $[\text{H}^+] = \sqrt{K_a C} = \sqrt{4.0 \times 10^{-10} \times 0.0025} = \sqrt{1.0 \times 10^{-12}} = 10^{-6}\text{ M}$. The pH is $-\log(10^{-6}) = 6$. Therefore, correct answer is 6.
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