We’re being asked to calculate the vapor pressure of water at 338K (64.85^oC) above a solution of a nonvolatile solute in water having a boiling point of 380.4K. (107.25^oC). 

The vapor pressure of pure water at this temperature is 0.2467 atm.

Recall that the boiling point of a solution is higher than that of the pure solvent and the change in boiling point (ΔT­_b) is given by:     

$$\begin{gathered} \boxed{\mathbf{∆}{\mathbf{T}}_{\mathbf{b}}\mathbf{ }\mathbf{=}\mathbf{ }{\mathbf{T}}_{\mathbf{b}\mathbf{,}\mathbf{ }\mathbf{solution}}\mathbf{ }\mathbf{-}\mathbf{ }{\mathbf{T}}_{\mathbf{b}\mathbf{,}\mathbf{ }\mathbf{pure}\mathbf{ }\mathbf{solvent}}} \\ \mathbf{∆}{\mathbf{T}}_{\mathbf{b}}\mathbf{=}\mathbf{ }\mathbf{107}\mathbf{.}\mathbf{25}\mathbf{ }{}^{\mathbf{o}}\mathbf{C}\mathbf{ }\mathbf{-}\mathbf{100}\mathbf{.}\mathbf{0}\mathbf{ }{}^{\mathbf{o}}\mathbf{C} \\ \mathbf{∆}{\mathbf{T}}_{\mathbf{b}}\mathbf{=}\mathbf{ }\mathbf{7}\mathbf{.}\mathbf{25}\mathbf{ }{}^{\mathbf{o}}\mathbf{C} \end{gathered}$$

The change in boiling point is also related to the molality of the solution:

$\boxed{\mathbf{∆}{\mathbf{T}}_{\mathbf{b}}\mathbf{ }\mathbf{=}\mathbf{ }{\mathbf{K}}_{\mathbf{b}}\mathbf{m}}$

where: 

m = molality of the solution (in m or mol/kg)

K_b = boiling point elevation constant (0.512 ˚C/m)