## Chemistry (4th Edition)

(b) $$K_a = 2.1 \times 10^{-6}$$
1. Write the equilibrium constant expression: - The exponent of each concentration is equal to its balance coefficient. $$K_c = \frac{[Products]}{[Reactants]} = \frac{[ H^+][ A^- ]}{[ HA ]}$$ 2. At equilibrium, these are the concentrations of each compound: $[ HA ] = 0.10 - x$ $[ H^+ ] = x$ $[ A^- ] = x$ Since $[H^+] = 4.6 \times 10^{-4}$, $x = 4.6 \times 10^{-4}$ $[HA] \approx 0.10 M$ $[H^+] = [A^-] = 4.6 \times 10^{-4} M$ $$K_a = \frac{(4.6 \times 10^{-4})(4.6 \times 10^{-4})}{(0.10)} = 2.1 \times 10^{-6}$$