Question

HZ is a weak acid. An aqueous solution of HZ is prepared by dissolving 0.020 mol of HZ in sufficient water to yield 1.0 L of solution.The pH of the solution was 4.93 at 25.0oC.The Ka of HZ is ________. A) 1.2×10−5B) 6.9×10−9C) 1.4×10−10D) 9.9×10−2E) 2.8×10−12

Answer 1

Answer:

B) 6.9 × 10⁻⁹

Explanation:

HZ is a weak acid that ionizes according to the following generic equation.

HZ(aq) ⇄ H⁺(aq) + Z⁻(aq)

First, we have to calculate the concentration of H⁺ using the pH.

pH = -log [H⁺]

[H⁺] = antilog -pH = antilog -4.93 = 1.17 × 10⁻⁵

Then, we have to calculate the initial concentration of the acid (Ca).

$Ca=\frac{0.020mol}{1.0L} =0.020M$

Finally, we can calculate the acid dissociation constant (Ka) using the following expression.

$Ka=\frac{[H^{+}]^{2} }{Ca} =\frac{(1.17 \times 10^{-5})^{2}   }{0.020} =6.9 \times 10^{-9}$