Question

A certain weak acid, HA, has a Ka value of 2.8×10−7. Calculate the percent ionization of HA in a 0.10 M solution. Calculate the percent ionization of HA in a 0.010 M solution.

Answer 1

Let a be the dissociation of the weak acid
                            HA <---> H ⁺ + A^-

initial conc.            c               0         0

Equb. conc.         c(1-a)          ca       ca

Dissociation constant , K_a = ca x ca / ( c(1-a)

                                         = c a² / (1-a)

In the case of weak acids α is very small so 1-a is taken as 1

So K_a = ca²

==> a = √ ( K_a / c )

Given K_a = 2.8x10^-7

          c = concentration = 0.10 M

Plug the values we get a = 1.673x10^-3

∴ % ionization =  1.673x10^-3 x 100 =  1.673x10^-1

------------------------------------------------------------------------------------------------------------

Calculation of percentage of ionization in 0.010 M solution :

a = √ ( K_a / c )

Given K_a = 2.8x10^-7

          c = concentration = 0.010 M

Plug the values we get a = 5.29x10^-3

∴ % ionization = 5.29x10^-3 x 100 = 0.529