The observed osmotic pressure for a 0.10 M solution of  $\mathrm{Fe}{\left({\mathrm{NH}}_4\right)}_2\left({\mathrm{SO}}_4\right)\text{ at }{25}^{\circ }\mathrm{C}$ 10.8 atm. The expected and experimental ( observed) values of vant6' Hoff Factor(i) will be respectively $\left(\mathrm{R}=0.082\mathrm{L}\text{ atm }{\mathrm{K}}^{-1}{\mathrm{mol}}^{-1}\right)$

$\mathrm{Fe}{\left({\mathrm{NH}}_4\right)}_2{\left({\mathrm{SO}}_4\right)}_2$ 2 ionizes as

$\mathrm{Fe}{\left({\mathrm{NH}}_4\right)}_2{\left({\mathrm{SO}}_4\right)}_2\to {\mathrm{Fe}}^{2+}+2{\mathrm{NH}}_4^{+}+2{\mathrm{SO}}_4^{2-}$

The number of species produced is 5 times than that of $\mathrm{Fe}{\left({\mathrm{NH}}_4\right)}_2{\left({\mathrm{SO}}_4\right)}_2$. Therefore, the expected van't Hoff factor is 5. 

The expected osmotic pressure assuming no ionization is

$\pi =cRT=\left(0.10{\mathrm{molL}}^{-1}\right)\left(0.082{\mathrm{LatmK}}^{-1}{\mathrm{mol}}^{-1}\right)\left(298.15\mathrm{K}\right)=2.45\mathrm{atm}$The observed van 't Hoff factor is

$i=\frac{10.8\mathrm{atm}}{2.45\mathrm{atm}}=4.4$