Solution I contains $6.0 g$ of urea (molar mass $60$ ) in $100 g$ of water. Solution II contains $18.0 g$ of a non-volatile non-electrolyte X in $100 g$ of water. The two solutions have the same vapour pressure at the same temperature. The molar mass of X in $g mo l^{- 1}$ is

Answer: D

Equal vapour pressure at equal solvent mass means equal moles of solute. Urea:

6.0/60 = 0.10 mol

. So X:

18.0/ M = 0.10

, giving

M = 180 g mo l^{- 1}