Question

Raoult's Law describes a simple relationship between the vapor pressure of an ideal solution and the vapor pressure of pure solvent. Non-ideal solutions, however, show deviation from this ideal behavior. Solutions with vapor pressures lower than predicted (negative deviation) are the result of?

Answer 1

If the inter molecular attractions forces in liquid A are equal (similar) to inter molecular forces in liquid B . Then A and B liquids will form an ideal solution. Because attractions forces between A and B molecules is similar to attraction forces between A-A molecules and atrraction forces between B-B molecules. that means strength of inter attraction forces are same in individual liquids and A-B mixture. Then vapor pressure of the solution is equal to summation of partial pressures of A and B liquids.

If the intermolecular forces in individual liquids are different from the molecular attraction forces in solution then we get deviations from the Raoult's law. That means vapor pressure of the solution is not equal to summation of partial pressures of liquids A and B.

If the vapor pressure of solution is less than the summation of partial pressures of A and B then it is called negative deviation. This is due to strong intermolecular attraction forces between A-B than the intermolecular forces between individual molecules (A-A and B-B).

Strength of intermolecular forces between A and B molecules > strength of intermolecular forces bwteen A-A or B-B