What is the ionic strength of a soil solution sample containing the following solute concentrations in mol L$-1$: : $[$Ca$2+]=2\mathrm{.}1$ x $10-3$; $[$Zn$2+]=1\mathrm{.}8$ x $10-5$; $[$Al$3+]=3\mathrm{.}2$ x $10-4$; $[$NO$3-]=2\mathrm{.}7$ x $10-3$; $[$Cl$-]=3\mathrm{.}7$ x $10-3.$ What is the activity coefficient for Ca$2+$ and the activity of Ca$2+$ in this solution? How does this differ from Al$3+$ and why does it differ