Question

How do you calculate the Ka of an acid?

Answer 1

`K_a` applies to weak acids in aqueous solution

Explanation:

For a hypothetical weak acid `HA rightleftharpoons H^+ + A^-`

`K_a = (([H^+][A^-])/[HA])` where `[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 = 0.0015M`
`[HA] ~~ 0.10M - 0.0015M~~ 0.0985M`

`K_a = (([H^+][A^-])/[[HA]]) = ((0.0015)(0.0015))/(0.0985)` = `2.28xx10^-4`