According to Raoult's Law

$${{{P^ \circ } - {P_s}} \over {{P_s}}} = {{{W_B} \times {M_A}} \over {{M_B} \times {W_A}}}\,\,\,\,\,\,\,\,\,\,...\left( i \right)$$

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 $${100^ \circ }C$$

(by assumption $$=760$$ torr)

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

$${{760 - {P_s}} \over {{P_s}}} = {{18 \times 18} \over {180 \times 178.2}}\,\,\,\,\,\,...\left( {ii} \right)$$

On solving $$(ii)$$ we get

On solving $$(ii)$$ we get

$${P_s} = 752.4\,\,torr$$