Vapour Pressure, Ideal Solutions, and Deviations

Identify whether the solute is volatile or non-volatile. For non-volatile solute, use solvent mole fraction to calculate vapour pressure. For deviation questions, compare A-B interactions with A-A and B-B interactions, then decide whether observed vapour pressure is higher or lower than ideal.

A vapour pressure versus mole fraction graph is important because exam questions commonly ask students to identify ideal line, positive deviation, and negative deviation from curves.

Taking x_solvent as relative lowering instead of x_solute gives the opposite numerical answer.

Vapour pressure of a solution is the pressure exerted by vapour in equilibrium with the liquid solution; addition of a non-volatile solute lowers the vapour pressure of the solvent, and liquid pairs may show ideal or non-ideal behaviour.

Vapour pressure of a solution is the pressure exerted by vapour in equilibrium with the liquid solution; addition of a non-volatile solute lowers the vapour pressure of the solvent, and liquid pairs may show ideal or non-ideal behaviour. For a non-volatile solute: p_solution = p_solvent^0 x_solvent. Relative lowering of vapour pressure: (p^0 - p)/p^0 = x_solute for dilute ideal solutions. Positive deviation: observed vapour pressure is higher than Raoult's law prediction. Negative deviation: observed vapour pressure is lower than prediction. A solution of sugar in water has lower vapour pressure than pure water because sugar is non-volatile and reduces the mole fraction of water at the surface.

For an ideal solution, each volatile component follows Raoult's law across the full composition range. When a non-volatile solute is added, fewer solvent molecules escape from the surface, so vapour pressure decreases. Non-ideal solutions show positive or negative deviation depending on whether unlike molecular interactions are weaker or stronger than like interactions. For a non-volatile solute: p_solution = p_solvent^0 x_solvent. Relative lowering of vapour pressure: (p^0 - p)/p^0 = x_solute for dilute ideal solutions. Positive deviation: observed vapour pressure is higher than Raoult's law prediction. Negative deviation: observed vapour pressure is lower than prediction. If pure solvent vapour pressure is 100 kPa and solvent mole fraction in solution is 0.80, then p_solution = 100 x 0.80 = 80 kPa. Relative lowering = (100 - 80)/100 = 0.20, equal to solute mole fraction for an ideal dilute case. Questions may ask for relative lowering calculation, graph interpretation, identification of positive or negative deviation, or reasoning based on intermolecular forces. Taking x_solvent as relative lowering instead of x_solute gives the opposite numerical answer.