#### Answer

186.4 torr

#### Work Step by Step

According to Raoult's law, VP(solution) = mole fraction * VP(pure solvent). First we need to convert 338K to Celsius, which we do by subtracting 273 to give 65C. From appendix B, we have that the vapor pressure of water is 187.5 torr. Then we need to find the mole fraction. We have 200g of water and the molar mass of water is 18 g/mol so that is 11.11 moles. Then we have 22.5g of lactose (C12H22O11), which has a molar mass of 342 g/mol so that is 0.0658 moles. Therefore, the mole fraction is 11.11 / (11.11 + 0.0658) = 0.994. Therefore, the vapor pressure would be 0.994 * 187.5 = 186.4 torr