The observed osmotic pressure for a $$0.10\,M$$  solution of $$Fe{\left( {N{H_4}} \right)_2}{\left( {S{O_4}} \right)_2}$$    at $${25^ \circ }C$$  is $$10.8\,atm.$$  The experimental (observed) value of van’t Hoff factor will be :
$$\left( {R = 0.082\,L\,atm\,{k^{ - 1}}\,mo{l^{ - 1}}} \right)$$

Solution :
$$\eqalign{ & {\text{Given}}\,\,{\pi _{ob}} = 10.8\,atm \cr & {\pi _{nor}} = CRT = 0.10 \times 0.0821 \times 298 = 2.446 \cr & {\text{now experimental value of }}(i) \cr & = \frac{{{\text{Observed osmotic pressure}}}}{{{\text{Normal osmotic pressure}}}} \cr & = \frac{{10.8}}{{2.446}} \cr & = 4.42 \cr} $$