An ideal solution is one in which the attraction between components of the solution is the same as the interaction between the molecules of each component. Heat is neither absorbed nor evolved during the formation of ideal solution and the volume of the solution is equal to the sum of the volumes of the component liquids. The vapour pressures of ideal solutions can be calculated by averaging the properties of the liquids. The solutions in which properties of dissolved liquids are different from those of the liquids in the pure state and which are formed by evolution or absorption of heat are called non-ideal solutions. Raoult’s law states that partial pressures of component (say liquid A) in solution is proportional to the mole fraction. If all the components in solutions behave like ideal gases, then the total pressure of the solution is equal to the sum of the partial pressure of the individual components.

\[P_{Total} = \chi _AP_A^o + \chi _BP_B^o\]

where \(P_A^o\) and \(P_B^o\) are the vapour pressures of pure solvents A and B, respectively, and \(\chi _A\) and \(\chi _B\) are mole fractions of the solvents A and B in solution. The composition of vapour of an ideal solution can be determined by the partial pressures of the components.

If \(Y_A\) and \(Y_B\) are the mole fractions of the components A and B in the vapour phase, the partial

vapour pressures of A and B can be calculated using Dalton’s law of partial pressures.

\[P_A = Y_AP_{Total}\]

\[P_B = Y_BP_{Total}\]

A non-ideal solution is that solution (i) which does not obey Raoult’s law (ii) for which \(\Delta V_{mix}\) is not zero and (iii) for which \(\Delta H_{mix}\) is not zero. In non-ideal solutions, the solute–solvent interactions are weaker or stronger than the solute–solute and solvent–solvent interactions. The non-ideal solutions in which solute–solvent interactions are weaker or stronger than the solute– solute or solvent–solvent show positive deviations from Raoult’s law.

Considering the formation, breaking and strength of hydrogen bond, predict which of the following mixtures will show a positive deviation from Raoult’s law?

Methanol and acetone

The answer is 1. Methanol and acetone.

Methanol and acetone are both polar molecules, but they have different hydrogen bonding capabilities. Methanol can form hydrogen bonds with itself, but acetone cannot. As a result, when methanol and acetone are mixed, the methanol molecules will form hydrogen bonds with each other, but the acetone molecules will not. This will cause the total vapor pressure of the solution to be lower than the vapor pressure of the pure solvents.

Here are some additional explanations:

Positive deviation from Raoult's law occurs when the vapor pressure of the solution is higher than the vapor pressure of the pure solvents. This happens when the solute-solute interactions are stronger than the solvent-solvent interactions.

Methanol is a polar molecule that contains a hydroxyl group (-OH). The hydroxyl group can form hydrogen bonds with other methanol molecules.

Acetone is a polar molecule that does not contain a hydroxyl group. As a result, acetone cannot form hydrogen bonds with other acetone molecules.

When methanol and acetone are mixed, the methanol molecules will form hydrogen bonds with each other. However, the acetone molecules will not interact with each other very strongly. This will cause the total vapor pressure of the solution to be higher than the vapor pressure of the pure solvents.