Question

A 0.200 M solution of a weak acid, HA, is 9.4% ionized. Using this information, calculate Ka for HA.

Answer 1

Answer – We are given, percent ionization = 9.4 % , [HA] = 0.200 M

We know formula for percent ionization,

Percent ionization = x/ initial concentration of HA *100 %

x = ionized species concentration = [H₃O⁺]

So,

10 % = x / 0.200 * 100 %

x = 9.4 %*0.200 / 100 %

    = 0.0188 M

X = [H₃O⁺] = [HA^-] = 0.0188 M

So, [HA] = 0.200 - 0.0188 M

                 = 0.181 M

We know,

Ka =[H₃O⁺][HA^-] / [HA]

     = 0.0188 *0.0188 / 0.181

     = 1.95*10^-3