Calculate the vapor pressure of water above a solution prepared by adding 21.5 g of lactose (C12H22O11) to 200.0 g of water at 338 K. (Vapor-pressure of water at 338 K 187.5 torr.)

The vapor pressure of the solution is 186.4 torr

Explanation:

As the relative lowering of vapor pressure is directly proportional to the amount of dissolved solute.

The formula for relative lowering of vapor pressure will be,

where,

= relative lowering in vapor pressure

i = Van'T Hoff factor = 1 (for non electrolytes)

= mole fraction of solute  =

Given : 21.5 g of lactose is present in 200.0 g of water.

moles of solute (lactose) =

moles of solvent (water) =

Total moles = moles of solute (lactose) + moles of solvent (water) = 0.0628+ 11.1 = 11.1628

= mole fraction of solute  =

Thus the vapor pressure of the solution is 186.4 torr

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