The activity of a solution is given by the expression: a $=c+-.$ Where the mean activity coefficient at $25C$ is given by the Debye Huckel limiting law as: $log{}^{+-}=-0\mathrm{.}5091|Z^{+}{Z}^{-}|\sqrt[\phantom{2}]{?}.$

The ionic strength is given by the expression: $=\frac{1}{2}({\displaystyle \underset{?}{\overset{?}{}}}{c}_i{Z}_i^2)$

Calculate the activity of the solution of a $0\mathrm{.}4MN{a}_{2}S{O}_4$ solution

Select one:

a$.0\mathrm{.}031$

b$.0\mathrm{.}039$

c$.0\mathrm{.}011$

d$.0\mathrm{.}0086$