Question

Question #a2b19

Answer 1

I think I know what your problem is..

Explanation:

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"`.