Sea water is $$3.5\% $$  by mass of common salt and has a density $$1.04\,g\,c{m^{ - 3}}$$   at $$293\,K.$$  Assuming the salt to be sodium chloride, then osmotic pressure of sea water will be ( assume complete ionisation of the salt )

Solution :
$$\eqalign{ & {\rm{Let}}\,\,{\rm{mass}}\,\,{\rm{of}}\,\,{\rm{solution}}\,\,{\rm{be}}\,\,{\rm{100 g}}{\rm{.}} \cr & {\rm{Volume}}\,\,{\rm{of}}\,\,{\rm{solution}} \cr & = {{{\rm{Mass}}} \over {{\rm{Density}}}} \cr & = {{100} \over {1.04}} \cr & = 96.154\,mL \cr & = 0.096\,L \cr} $$
$${w_B} = 3.5\,g;{M_B} = 58.5\,g\,mo{l^{ - 1}}$$      ( molar mass of $$NaCl$$  )
For complete ionisation of $$NaCl,$$  van't Hoff factor will be $$'2'.$$
$$\eqalign{ & i = 2, \cr & {\rm{we}}\,\,{\rm{know,}} \cr & \pi = iCRT \cr & \,\,\,\, = i{n \over V}RT \cr & \,\,\, = 2 \times {{3.5 \times 0.0821 \times 293} \over {58.5 \times 0.096}} \cr & \,\,\, = 29.98\,atm \cr} $$