Relative Lowering of Vapour Pressure — Core Principles

Core Principles

Vapour pressure is the pressure exerted by the vapour in equilibrium with its liquid phase. When a non-volatile solute is added to a pure solvent, it occupies some surface area, reducing the number of solvent molecules that can escape into the vapour phase.

This leads to a decrease in the solvent's vapour pressure, known as 'lowering of vapour pressure'. The 'relative lowering of vapour pressure' (RLVP) is the ratio of this lowering to the vapour pressure of the pure solvent.

According to Raoult's Law, for ideal dilute solutions, RLVP is directly equal to the mole fraction of the solute ($X_{solute}$). Mathematically, it's expressed as $\frac{P^0 - P_s}{P^0} = X_{solute}$, where $P^0$ is the vapour pressure of the pure solvent and $P_s$ is the vapour pressure of the solution.

This property is colligative, meaning it depends only on the number of solute particles, not their identity, and is crucial for determining the molar mass of unknown non-volatile solutes.

Often confused with

Side-by-side differences the NEET paper likes to test.

Relative Lowering of Vapour Pressure vs Elevation of Boiling Point (EBP)

Aspect	Relative Lowering of Vapour Pressure	Elevation of Boiling Point (EBP)
Definition	Relative Lowering of Vapour Pressure (RLVP) is the fractional decrease in the vapour pressure of a solvent upon addition of a non-volatile solute.	Elevation of Boiling Point (EBP) is the increase in the boiling point of a solvent upon addition of a non-volatile solute.
Direct Cause	Reduced number of solvent molecules at the liquid surface available for evaporation.	Lowering of vapour pressure, which means a higher temperature is needed for the solution's vapour pressure to reach atmospheric pressure.
Formula	$\frac{P^0 - P_s}{P^0} = X_{solute}$ (or $i \cdot X_{solute}$)	$\Delta T_b = K_b \cdot m$ (or $i \cdot K_b \cdot m$)
Concentration Term	Mole fraction ($X_{solute}$)	Molality ($m$)
Measurement	Requires precise measurement of vapour pressures of pure solvent and solution.	Requires precise measurement of boiling points of pure solvent and solution.
Relationship	RLVP is the fundamental colligative property from which EBP (and others) are derived.	EBP is a direct consequence of RLVP; a lower vapour pressure implies a higher boiling point.

Both Relative Lowering of Vapour Pressure (RLVP) and Elevation of Boiling Point (EBP) are colligative properties, meaning they depend on the number of solute particles, not their identity. However, they describe different observable phenomena and use different concentration units in their primary formulas.

RLVP quantifies the fractional decrease in vapour pressure due to a non-volatile solute, expressed in terms of mole fraction. EBP quantifies the increase in boiling temperature, expressed in terms of molality.

Fundamentally, RLVP is the root cause for EBP; a solution with lower vapour pressure needs to be heated to a higher temperature to achieve the same vapour pressure as the pure solvent at its boiling point.

Why it is tested: For NEET, understanding the distinction and interrelationship between RLVP and EBP is crucial. Questions often test the conceptual link, asking which solution will have the lowest vapour pressure and consequently the highest boiling point. Students must be able to apply both formulas and recognize when to use mole fraction versus molality, and how the van't Hoff factor applies to both for electrolytes.