$\frac{P^{\circ}-P_{s}}{P_{s}}=\frac{W_{B} \times M_{A}}{M_{B} \times W_{A}} \quad \ldots(i)$

Here $P^{\circ}=$ Vapour pressure of pure solvent,

$P_{s}=$ Vapour pressure of solution

$W_{B}=$ Mass of solute,

$W_{A}=$ Mass of solvent

$M_{B}=$ Molar mass of solute,

$M_{A}=$ Molar Mass of solvent

Vapour pressure of pure water at $100\,{\,^o}C$

(by assumption $=760\,torr$ )

By substituting values in equation $(i)$ we get,

$\frac{760-P_{x}}{P_{x}}=\frac{18 \times 18}{180 \times 178.2} \quad \ldots(i i)$

On solving $(i i)$ we get

$P_{s}=752.4\,torr$