An ion  Mn^a+ has the magnetic moment equal to 4.9BM. what is the value of a?

$\because \text{ Magnetic moment }=\sqrt{\mathrm{n}(\mathrm{n}+2)}=4.9\mathrm{B}\cdot \mathrm{M}$

$\therefore \mathrm{n}=4$ (where n is no. of unpaired electron)

Thus  Mn^a+ ion has four unpaired electron,

$$\begin{gathered} { }_{25}{\mathrm{Mn}}^{3+}:1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}3{\mathrm{d}}^4 \\ \therefore \mathrm{a}=3 \end{gathered}$$