How do you calculate the Ka of an acid?

1 Answer
Doc048
Jul 10, 2017

K_aKa applies to weak acids in aqueous solution

Explanation:

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

K_a = (([H^+][A^-])/[HA])Ka=([H+][A−]HA) where [H^+], [A^-] & [HA][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[H+]=[A_]=0.015(0.10)M=0.0015M
[HA] ~~ 0.10M - 0.0015M~~ 0.0985M[HA]≈0.10M−0.0015M≈0.0985M

K_a = (([H^+][A^-])/[[HA]]) = ((0.0015)(0.0015))/(0.0985)Ka=([H+][A−][HA])=(0.0015)(0.0015)0.0985 = 2.28xx10^-42.28×10−4

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