Question

Thioglycolic acid, a substance used in depilatory agesnt has pKa=3.42. What is the percent dissociation of thioglycolic acid in a buffer solution at pH=3.0? Please offer a good explanation!

Answer 1

calculate the ka value from pKa

ka = 10^-pKa  = 10^-3.42  = 3.8 x 10^-4

Thioglycolic acid is a HSCH2COOH it is a week acid

with a given information we can find out the concentration

pH = 1/2 (pKa-logC)

where C is the concentration of thioglcolic acid

3.0 = 1/2 (3.42 - logC)

logC = 3.42-6

logC = -2.58

C = 10^-2.58

C = 0.00263M

now construct the ICE table

HSCH2COOH <-----------> HSCH2COO- + H+

I 0.00263 0 0

C -x +x +x

E 0.00263-x +x +x

Ka = [H+][HSCH2COO-] / [HSCH2COOH]

3.8 x 10^-4 = [x][x] /[0.00263-x]

x² + x3.8 x 10^-4 -9.9 x 10^-7 =0

solve the quadratic equation

x = 0.000827 M= [H+]

now percentage of ionisation = (concentration of H+ / concemtration of Thioglycolic acid) x 100

= (0.000827M / 0.00263M) x 100

= 31.44 %