Magnetic moment of a salt depends upon the number of unpaired d-electrons. In $\mathrm{Zn}^{2+}$ salt configuration of cation is $4 \mathrm{~s}^{0} 3 \mathrm{~d}^{10}$. Hence total no. of unpaired electron, $\mathrm{n}$ is zero. So magnetic moment i.e.
$\text { B.M. }=\sqrt{\mathrm{n}(\mathrm{n}+2)}=0$