The partial pressure of a volatile component of a solution is equal to the mole fraction of that component times the vapor pressure of that component. This is known as Raoult's Law.
In a solution of alcohol in water (VP stands for vapor presure),
Partial pressure of alcohol = mole fraction of alcohol x VP of alcohol Partial pressure of water = mole fraction of water x VP of water VP of the solution = VP of water + VP of alcohol
In a solution of sugar in water,
Partial pressure of water = mole fraction of water x vapor pressure of water
The vapor pressure of sugar is so low that it is not significant. (Sugar is considered nonvolatile.)
This relationship refers to ideal solutions where the interactions between solvent and solute molecules are identical. In most real solutions, the solute molecules do interact with the solvent molecules more strongly than with each other. These interactions result in small deviations from this law.
Raoult's Law and colligative properties are reviewed several places on the WWW, and are topics in the course on Water and Solutions.
