Editing Colligative properties (section)

==Relative lowering of vapor pressure==
A [[vapor]] is a substance in a gaseous state at a temperature lower than its [[Critical point (thermodynamics)|critical point]]. [[Vapor pressure]] is the pressure exerted by a vapor in thermodynamic equilibrium with its solid or liquid state. The vapor pressure of a solvent is lowered when a non-volatile solute is dissolved in it to form a solution.

For an [[ideal solution]], the equilibrium vapor pressure is given by [[Raoult's law]] as
<math display="block">p = p^{\star}_{\rm A} x_{\rm A} + p^{\star}_{\rm B} x_{\rm B} + \cdots,</math>
where <math>p^{\star}_{\rm i}</math> is the vapor pressure of the ''pure'' component (i= A, B, ...) and <math>x_{\rm i}</math> is the [[mole fraction]] of the component in the solution.

For a solution with a solvent (A) and one non-volatile solute (B),  <math>p^{\star}_{\rm B} = 0</math> and <math>p = p^{\star}_{\rm A} x_{\rm A}</math>.

The vapor pressure ''lowering'' relative to pure solvent is <math>\Delta p = p^{\star}_{\rm A} - p = p^{\star}_{\rm A} (1 - x_{\rm A}) = p^{\star}_{\rm A} x_{\rm B}</math>, which is proportional to the mole fraction of solute.

If the solute [[ionic dissociation|dissociates]] in solution, then the number of moles of solute is increased by the [[van 't Hoff factor]] <math>i</math>, which represents the true number of solute particles for each formula unit. For example, the [[strong electrolyte]] [[Magnesium chloride|MgCl<sub>2</sub>]] dissociates into one Mg<sup>2+</sup> ion and two Cl<sup>−</sup> ions, so that if ionization is complete, i = 3 and <math>\Delta p = p^{\star}_{\rm A} x_{\rm B}</math>, where <math>x_{\rm B}</math> is calculated with moles of solute i times initial moles and moles of solvent same as initial moles of solvent before dissociation. The measured colligative properties show that ''i'' is somewhat less than 3 due to [[ion association]].