When a certain amount of solid A is dissolved in 100   g of water at 25   ° C to make a…

We know that relative lowering in vapour pressure is given as

$\frac{{\mathrm{P}}^0-{\mathrm{P}}_{\mathrm{S}}}{{\mathrm{P}}^{\mathrm{o}}}=\frac{{}^{\mathrm{n}}{\displaystyle \mathrm{solute}}}{{\mathrm{n}}_{\mathrm{Solute}}+{}^{\mathrm{n}}\mathrm{solvent}}$

$\frac{{\mathrm{P}}^0-{\mathrm{P}}_{\mathrm{S}}}{{\mathrm{P}}_{\mathrm{S}}}=\frac{{}^{\mathrm{n}}{\displaystyle \mathrm{solute}}}{{}^{\mathrm{n}}\mathrm{solvent}}$

$\frac{{\displaystyle {\mathrm{P}}^0-\frac{{\mathrm{P}}^0}{2}}}{{\displaystyle \frac{{\mathrm{P}}^0}{2}}}=\frac{{}^{\mathrm{n}}\mathrm{solute}}{{}^{\mathrm{n}}\mathrm{solvent}}$

${}^{\mathrm{n}}\mathrm{solute}={}^{\mathrm{n}}\mathrm{solvent}=\frac{100}{18}=5.55 \mathrm{mol}$