You are a chemical engineer at a processing plant for soft drinks. You have been asked to determine whether a particular solution will have a significantly different vapor pressure from pure water. Calculate the vapor pressure of water at 20°C in a solution prepared by dissolving 10.00 g of sucrose, C₁₂H₂₂O₁₁, in 100.0 g of water.

ANTICIPATE Expect a lower vapor pressure when the solute is present. However, the vapor pressure is not lowered very much by a solute, so you should expect a value slightly below that of pure water, which Table 5A.2 gives as 17.54 Torr at 20°C.

PLAN Calculate the mole fraction of the solvent (water) in the solution and then apply Raoult’s law. To use Raoult’s law, you need to know the vapor pressure of the pure solvent at the temperature of interest (Table 5A.1 or 5A.2).

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TABLE 5A.1 Vapor Pressures at 25 °C Substance benzene ethanol mercury methanol toluene water* Vapor pressure P/Torr 94.6 58.9 0.0017 122.7 29.1 23.8