# Vapour Pressure and Raoult's Law

> Raoult's law numericals for NEET Chemistry: partial vapour pressures, vapour composition by Dalton's law and positive-negative deviation azeotropes.

- Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/vapour-pressure-raoult-neet
- Exam / course: NEET-UG · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Vapour Pressure and Raoult's Law", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/vapour-pressure-raoult-neet

## Direct answer

Raoult's law links the vapour pressure above a liquid mixture to composition: for each volatile component, p(A) = p°(A) × x(A), the pure-component vapour pressure scaled by its mole fraction, and the total pressure is the sum of the partials. The vapour phase is always richer in the more volatile component, because y(A) = p(A)/P(total) weights each component by its own pressure. Ideal solutions — benzene with toluene — obey the law across all compositions with zero heat and volume of mixing; real ones deviate, and the deviation direction names the pair: acetone with chloroform shows negative deviation through hydrogen bonding, ethanol with acetone shows positive deviation, with minimum- and maximum-boiling azeotropes (about 95% ethanol-water, 20.2% HCl-water) as the practical consequences.

## What you must remember

- **Two working equations:** p(A) = p°(A)x(A) and P(total) = p(A) + p(B); vapour composition y(A) = p(A)/P(total).
- **Ideal-solution signatures:** ΔH(mix) = 0, ΔV(mix) = 0, Raoult obeyed over the full range; model pairs benzene + toluene, n-hexane + n-heptane.
- **Positive deviation pairs:** acetone + ethanol, carbon disulphide + acetone — A–B attraction weaker than self-attraction, vapour pressure above ideal, minimum-boiling azeotrope (ethanol-water at about 95% ethanol by volume).
- **Negative deviation pairs:** chloroform + acetone (hydrogen bond between them), nitric acid + water — vapour pressure below ideal, maximum-boiling azeotrope (HCl-water at 20.2% HCl, boiling near 383 K).
- **Non-volatile solute form:** relative lowering (p° − p)/p° = x(solute) — a colligative property depending only on solute particle count.
- **Liquid-versus-vapour comparison:** y exceeds x for the more volatile component always; computing y as if it equalled x is the standard error.
- **Henry's law bridge:** gases in liquids follow p = K(H)x; higher K(H) means lower solubility — the same partial-pressure logic wearing solution clothes.

## Two compositions worked with one data set

Liquids A (p° = 450 mm) and B (p° = 700 mm) mix ideally. Equimolar liquid: p(A) = 225 mm, p(B) = 350 mm, total 575 mm. Vapour composition: y(A) = 225/575 = 0.39, y(B) = 0.61 — the vapour is richer in B, the more volatile liquid, even though the liquid is half and half. That single comparison is the concept the examiner keeps testing: the vapour prefers the lower-boiling component.

Now invert the question — the total pressure is measured 600 mm; find the liquid composition. Solve 450x(A) + 700(1 − x(A)) = 600: 250x(A) = 100, x(A) = 0.40. Then y(A) = 0.40 × 450/600 = 0.30. Two equations, no chemistry beyond Raoult and Dalton, and both answers come out as clean fractions — which is exactly how the numerical is set.

## Phase and deviation traps

Phase confusion leads: y is not x, and which one the question asks for is stated in one word, "vapour" or "solution". Deviation direction runs second: decide whether the new A–B interaction is stronger or weaker than each liquid's self-interaction — chloroform's hydrogen bond to acetone's carbonyl tightens the mixture (negative deviation, higher boiling), while acetone breaking ethanol's own hydrogen network loosens it (positive deviation, lower boiling). Azeotrope questions demand the definition verbatim: a constant-boiling mixture whose distillate has the same composition as the liquid, which is why 95% ethanol cannot be pushed to 100% by simple distillation. And in relative-lowering problems, the mole fraction in the formula belongs to the solute, not the solvent — the inversion is quietly offered as a distractor.

## Frequently asked questions

### What is Raoult's law for two volatile components?

Each component's partial vapour pressure equals its pure vapour pressure times its mole fraction in the solution, p(i) = p°(i)x(i); total pressure is their sum.

### Why is the vapour richer in the more volatile component?

Because y(i) = p(i)/P(total) weights each component by its own pure vapour pressure, and the more volatile one contributes disproportionately.

### Why does chloroform-acetone show negative deviation?

They form a hydrogen bond (Cl–H···O=C) tighter than either liquid's self-interaction, lowering escaping tendency and vapour pressure below ideal.

### What is an azeotrope and why can it not be separated by distillation?

A constant-boiling mixture of fixed composition — the vapour and liquid compositions match, so distillation reproduces the same mixture endlessly, as with 95% ethanol-water.

### Why is relative lowering of vapour pressure colligative?

The equation (p° − p)/p° = x(solute) contains only the solute's mole fraction — particle count, not identity.