Question

A certain weak acid, HA, has a K_a value of 6.9x10^-7

1) Calculate the percent ionization of HA in a 0.10 M solution

Express your answer to two significant figures and include the appropriate units.

Value = Units =

2) Calculate the percent ionization of HA in a 0.010 M solution

Value = Units =

Answer 1

1)
HA < -- > H+ + A-
0.10 0 0 intial
0.10-x x x at equlibrium

since, x is very small for weak acid
0.10-x = 0.10

Ka = ([H+]*[A-])/[HA]
6.9*10^-7 = x^2/0.10
x^2 = 6.9*10^-8
x = 2.6*10^-4

%ionisation = {x/[HA]intial}*100
= {(2.6*10^-4)/0.10}*100
= 0.26% M-1

2)
HA < -- > H+ + A-
0.010 0 0 intial
0.010-x x x at equlibrium

since, x is very small for weak acid
0.010-x = 0.010

Ka = ([H+]*[A-])/[HA]
6.9*10^-7 = x^2/0.010
x^2 = 0.69*10^-8
x = 0.83*10^-4

%ionisation = {x/[HA]intial}*100
= {(0.83*10^-4)/0.010}*100
= 0.83% M-1