Relative Lowering Of Vapour Pressure

The pressure imposed by a substance’s vapour on its fluid/solid state in a sealed container at optimum conditions is known as its vapour pressure. Vapour pressure reduction is a colligative feature of fluids. A pure solvent’s vapour pressure is greater than a nonvolatile liquid solution’s vapour pressure. The boiling point rises as the vapour pressure falls. Let’s study everything there is to know about the relative lowering of vapour pressure in this article.

Vapour Pressure

The pressure imposed by a substance’s vapour on its fluid/solid state in a sealed container at optimum conditions is known as its vapour pressure. The evaporation of a liquid/ solid sample produces vapour. This vapour pressure of the liquid is in balance with its solid/liquid state at a particular temperature.

Vapour Pressure Influencing Factors

The following are the elements that influence vapour pressure:

1. Area of the Surface

The vapour pressure is unaffected by the amount of liquid / solid surface area in contact with the gas. This is because vapour pressure is a molecular attribute.

2. Types of molecules

The vapour pressure of a solid or liquid is determined by the types of molecules which make it up. The vapour pressure would be low if the intermolecular interactions between molecules are: 

• Relatively high.
• Because the substance is weak, the vapour pressure would be high.

3. Temperature

Many molecules would have enough heat energy to escape from the liquid state/solid-state to the gaseous state as the temperature increases. Only a few molecules have enough energy to transition from a liquid /solid to a gaseous form at a lower temperature.

Relative Lowering Of Vapour Pressure

Dissolving a nonvolatile solute inside a pure solvent tends to lower the solvent’s vapour pressure as per the proportionate reduction of vapour pressure. If ps is the vapour pressure of the solutions and p is the vapour pressure of the solvent, lowering the vapour pressure is expressed as (p – ps). The relative decrease of vapour pressure refers to the reduction in vapour pressure to that of the solvent.

Raoult’s Law

Raoult proposed an empirical relationship in 1886 to establish a link between the relative decrease of vapour pressure and the concentration of solutes in a solution, which came to be known as Raoult’s law  According to this law, the mole fraction of solute in a dilute solution equals the relative decrease in vapour pressure.

The evaporating molecule from the surface causes the vapour pressure of the pure solvent. Because of the presence of solute molecules on the surface, whenever a nonvolatile solute is dissolving in solution, the surface becomes clogged and also no operation occurs.

The lowering of vapour pressure by a nonvolatile solute prevents the solvent molecules from escaping the solution, implying that

N / (n+N) is directly proportional to Ps.

Here, N is the number of moles of solvent, and n is the number of moles of solute.

(p – ps) / p = n / (n + N) is the equation for Raoult’s law.

Deviance from Raoult’s Law and the Ideal Solution

The best solutions are those that adhere to Raoult’s Law. Assume that a & b denote the solvent and solute molecules, respectively. Let λAB