Question #a2b19

Raoult's law indeed states that the partial pressure of a gas varies directly as its mole fraction, #chi#:

#P_ "a" = (chi_ "a")(P_ "a")#

where

• #P_ "a"# is the partial pressure of component #"a"# of the gaseous mixture (which is above the solution),

• #chi_ "a"# is the mole fraction of #"a"# in the solution, and

• #P_ "a"# is the pure vapor pressure of #"a"#

(This is a result of the colligative property called vapor-pressure lowering)

In your problem, it said that as concentration increases, the mole fraction increases, and the partial pressure decreases. The #color(red)("concentration"# means the concentration of the nonvolatile #color(red)("solute"#, not the concentration of the solvent (water). As the concentration of the solute increases, #color(red)("the mole fraction of the solute increases"#, and #color(blue)("the mole fraction of the solvent decreases"#. If the mole fraction of the solvent decreases, then #color(blue)("the vapor pressure of the solution decreases"#.

This may have been a result of confusion, but #color(green)("when the mole fraction/concentration of the solute increases,"#
#color(green)("the vapor pressure of the solution decreases"#.

That is, #color(green)("the partial pressure of the solvent decreases"#.