Question

Calculate the activity coefficient, ?, of Cu2 when the ionic strength of the solution, ? , is 0.078 M:

(a) by linear interpolation of the data in the table to the right;

yCu2+ = 0.440

(b) by using the extended Debye-Huckel equation at 25°C, where the ion size is 600 pm.

yCu2+ =

answer to part a is correct.

ionic strength ( $\mu$ ,M)	activity coeffient $\gamma$ Cu2+
0.001	0.870
0.005	0.749
0.01	0.675
0.05	0.485
0.1	0.405

Answer 1

For b.

The extended Debye-Hückel equation

Log y = (-0.51 · z² · u^1/2) / (1+ 3.3 · a · u^1/2 )

          = (-0.51 · 2² · 0.078^1/2) / (1+ 3.3 · 0.6 · 0.078^1/2 )

          = - 0.01241 / 1.01204

          = - 0.01226

        y = 10^{- 0.01226} = 0.972

Note:

For the effective diameter a, use the value in nm for calculation:

         600 pm = 0.600 nm