$\frac{ P ^{0}- P _{ s }}{ P ^{0}} \sim \frac{{ }^{n} \text { solute }}{{ }^{n} \text { solvent }}$
$\frac{ P ^{0}- P ^{0} / 2}{ P ^{0}}=\frac{{ }^{n} \text { solute }}{ n _{\text {solvent }}}$
" solute $\sim \frac{{ }^{ n } \text { solvent }}{2}=\frac{100}{18 \times 2}=2.78 mol$
More accurate approach:
$\frac{ P ^{0}- P _{ S }}{ P _{ S }}=\frac{{ }^{ n } \text { solute }}{{ }^{n} \text { solvent }}$
$\frac{ P ^{0}- P ^{0} / 2}{ P ^{0} / 2}=\frac{{ }^{n} \text { solute }}{{ }^{n} \text { solvent }}$
$\text { " solute }={ }^{n} \text { solvent }=\frac{100}{18}=5.55 mol$
($k _{ f }=1.86\,K\,kg\,mol ^{-1}$ )
$(i)$ $0.10\, {M} \,{Ba}_{3}\left({PO}_{4}\right)_{2}$
$(ii)$ $0.10\, {M}\, {Na}_{2} {SO}_{4}$
$(iii)$ $0.10\, {M}\, {KCl}$
$(iv)$ $0.10 \,{M} \,{Li}_{3} {PO}_{4}$