The temperature at which molarity of pure water is equal to its molality is `:`

Options

A. 273 K
B. 298 K
C. 277 K
D. None of these

Answer

C

Explanation

Molality and mole fraction are related as follows:
`m = (x_(B) xx 1000)/((1 - x_(B)) m_(A))`
`1 = (x_(B) xx 1000)/((1 - x_(B)) xx 18)`
[ `m = 1 x_(B) =` mole fraction of solute `m_(A)=` molar mass of solvent]
`x_(B) = 0.0176, " " x_(A) = 0.9824`
`p = p_(0) x_(A)`
`p = 760 xx 0.9824`
`= 746.624`
`Delta p = P_(0) - p = 760 - 746.624`
`~~ 13.4` mm Hg