$\frac{ P ^{0}- P _{ s }}{ P ^{0}} \sim \frac{{ }^{n} \text { solute }}{{ }^{n} \text { solvent }}$
$\frac{ P ^{0}- P ^{0} / 2}{ P ^{0}}=\frac{{ }^{n} \text { solute }}{ n _{\text {solvent }}}$
" solute $\sim \frac{{ }^{ n } \text { solvent }}{2}=\frac{100}{18 \times 2}=2.78 mol$
More accurate approach:
$\frac{ P ^{0}- P _{ S }}{ P _{ S }}=\frac{{ }^{ n } \text { solute }}{{ }^{n} \text { solvent }}$
$\frac{ P ^{0}- P ^{0} / 2}{ P ^{0} / 2}=\frac{{ }^{n} \text { solute }}{{ }^{n} \text { solvent }}$
$\text { " solute }={ }^{n} \text { solvent }=\frac{100}{18}=5.55 mol$