# Question a2b19

Jun 7, 2017

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

• ${P}_{\text{a}}$ is the partial pressure of component $\text{a}$ of the gaseous mixture (which is above the solution),

• ${\chi}_{\text{a}}$ is the mole fraction of $\text{a}$ in the solution, and

• ${P}_{\text{a}}$ is the pure vapor pressure of $\text{a}$

(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"#.