AP Chemistry / Unit 8: Acids and Bases
8.3 Weak Acid and Base Equilibria
Notes
Weak acids only partly ionize: $HA\rightleftharpoons H^++A^-$, $\displaystyle K_a=\frac{[H^+][A^-]}{[HA]}$.
- $\displaystyle [H^+]\approx\sqrt{K_a C}$.
- For conjugate pairs, $K_aK_b=K_w$.
Practice
$K_a=1\times10^{-5}$, $C=0.1$ M. $[H^+]\approx$
$\displaystyle \sqrt{10^{-6}}$.
That solution's pH:
$-\log10^{-3}$.
$K_a=1\times10^{-5}$. $K_b$ of its conjugate base:
$\displaystyle \frac{10^{-14}}{10^{-5}}$.
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