How do you calculate the Ka of an acid?

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How do you calculate the Ka of an acid?

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Answer 1 · 2017-07-10T04:26:28

$K_{a}$ $K_a$ applies to weak acids in aqueous solution

Explanation:

For a hypothetical weak acid $H A ⇌ H^{+} + A^{-}$ $HA rightleftharpoons H^+ + A^-$

$K_{a} = (\frac{[H^{+}] [A^{-}]}{H A})$ $K_a = (([H^+][A^-])/[HA])$ where $[H^{+}], [A^{-}] & [H A]$ $[H^+], [A^-] & [HA]$ are molar concentrations of hydronium ion, conjugate base and weak acid at equilibrium.

Example:
Given a 0.10M weak acid that ionizes ~1.5%
$[H^{+}] = [A^{_}] = 0.015 (0.10) M = 0.0015 M$ $[H^+] = [A^_] = 0.015(0.10)M = 0.0015M$
$[H A] \approx 0.10 M - 0.0015 M \approx 0.0985 M$ $[HA] ~~ 0.10M - 0.0015M~~ 0.0985M$

$K_{a} = (\frac{[H^{+}] [A^{-}]}{[H A]}) = \frac{(0.0015) (0.0015)}{0.0985}$ $K_a = (([H^+][A^-])/[[HA]]) = ((0.0015)(0.0015))/(0.0985)$ = $2.28 \times 10^{- 4}$ $2.28xx10^-4$

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How do you calculate the Ka of an acid?

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