Weak Acid Equilibrium

Question

For a weak acid HA with Ka = 4 × 10⁻⁵ and concentration 0.1 M, the degree of ionization is approximately:

RankGuru · Accepted Answer

For a **Weak Acid ($HA$)**:
$HA ightleftharpoons H^+ + A^-$
- Let $C$ be concentration and $\alpha$ be the degree of ionization.
- $K_a = \frac{C\alpha^2}{1-\alpha}$. For $\alpha \ll 1$, then $K_a \approx C\alpha^2$.
Calculation:
$\alpha = \sqrt{\frac{K_a}{C}} = \sqrt{\frac{4 	imes 10^{-5}}{0.1}} = \sqrt{4 	imes 10^{-4}}$
$\alpha = 2 	imes 10^{-2} = 0.02$
- This represents $2\%$ ionization.

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Weak Acid Equilibrium

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