Answer – We are given molality of
Na3PO4 = 0.0350 m
The mean activity coeffiecient , γ+/- = 0.685
We know, there are 4 ions in the Na3PO4, 3
for Na+ and 1 for PO43- and we
know formula for m+/-
So, m+/- = (m+
*m-)1/4* m
             Â
= (33 * 11)1/4 * 0.0350 m
             Â
= 0.0798
We know, ionic activity, a+/- =
γ+/- * m+/-
   Â
                                         =
0.685 * 0.0798
                                             Â
= 0.0547
