A 0.10 M solution of a weak acid, HA, is found to be 1.50% ionized. Calculate Ka for this acid.
(a) 1.5 (b) 2.3 x 10-1 (c) 2.3 x 10-5 (d) 2.3 x 10-6
(e) none of the above
Chemistry · Sun Jul 10 2022
Answered on
To calculate Ka for the weak acid, we need to first understand the concept of percent ionization and how it relates to the equilibrium constant.
Percent ionization is a measure of how much of the weak acid molecule dissociates into ions in solution. It is calculated by dividing the concentration of the ionized acid by the initial concentration of the weak acid and multiplying by 100.
Given that the 0.10 M solution of the weak acid is 1.50% ionized, we can calculate the concentration of the ionized acid by multiplying the percent ionization by the initial concentration:
Concentration of ionized acid = (1.50/100) x 0.10 M = 0.0015 M
Since the weak acid dissociates into its ions according to the equation HA ⇌ H+ + A-, the concentration of the weak acid at equilibrium is equal to the initial concentration minus the concentration of the ionized acid:
Concentration of weak acid at equilibrium = 0.10 M - 0.0015 M = 0.0985 M
The equilibrium constant (Ka) for the acid dissociation reaction is given by the equation:
Ka = [H+][A-] / [HA]
Substituting the concentrations we just calculated:
Ka = (0.0015 M)(0.0015 M) / 0.0985 M = 2.3 x 10^-5
Therefore, the correct option is (c) 2.3 x 10^-5.