Raoult’s Law Calculator

Calculate vapor pressure above an ideal solution using Raoult’s Law. Get partial pressures, total vapor pressure, vapor composition (y-values), and a clean mini visual — with optional step-by-step.

Background

For an ideal solution, each component’s partial vapor pressure is proportional to its mole fraction: P_i = x_i·P_i^*. Total pressure is the sum of partial pressures: P_total = P_A + P_B. Vapor composition comes from Dalton’s Law: y_A = P_A/P_total.

Enter values

What do you want to calculate?

Binary ideal solution (A + B volatile): P_A, P_B, P_total, y_A, y_B

Nonvolatile solute (vapor pressure lowering): P_solution, ΔP

Pure-component vapor pressures (same units)

Enter values like 95 or 95.2. Units can be mmHg, kPa, atm — just be consistent.

Composition in the liquid

Provide x_A (0–1). We’ll compute x_B=1-x_A. If you enter both, we can optionally normalize.

Normalize xA and xB if xA + xB ≠ 1

Optional labels (for nicer output)

Options:

Show step-by-step

Show mini visual

Quick picks:

Chips prefill inputs and calculate immediately.

Result:

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How to use this calculator

Pick a mode: binary ideal solution or nonvolatile solute.
Enter vapor pressures P^* and composition (x).
Click Calculate to get partial pressures, total pressure, and vapor composition (y).
Use Quick picks for instant examples.

How this calculator works

Binary mode uses P_A=x_AP_A^* and P_B=x_BP_B^*.
Total pressure: P_total=P_A+P_B.
Vapor composition: y_A=P_A/P_total, y_B=P_B/P_total.
Nonvolatile mode computes P=x_solventP_solvent^*, then ΔP=P^*-P.

Formula & Equation Used

Raoult’s Law: P_i = x_i·P_i^*

Total pressure: P_total = ΣP_i

Dalton’s Law (vapor composition): y_i = P_i/P_total

Vapor pressure lowering: ΔP = P^* − P

Example Problem & Step-by-Step Solution

Example 1 — Binary ideal solution (A + B volatile)

A liquid solution contains x_A=0.40 of A and x_B=0.60 of B. The pure-component vapor pressures are P_A^*=95 mmHg and P_B^*=28 mmHg. Find P_A, P_B, P_total, and vapor composition y_A, y_B.

Compute partial pressures using Raoult’s Law: P_A=x_AP_A^*, P_B=x_BP_B^*.
P_A = 0.40 × 95 = 38 mmHg
P_B = 0.60 × 28 = 16.8 mmHg
Total vapor pressure: P_total = P_A + P_B = 38 + 16.8 = 54.8 mmHg
Vapor composition from Dalton’s Law: y_A = P_A/P_total, y_B = P_B/P_total
y_A = 38/54.8 ≈ 0.693, y_B = 16.8/54.8 ≈ 0.307
Notice how the vapor is richer in the more volatile component (higher P^*).

Example 2 — Nonvolatile solute (vapor pressure lowering)

A solvent has P_solvent^*=23.8 mmHg. In solution, x_solvent=0.92. Find the solution vapor pressure P and the lowering ΔP.

Apply Raoult’s Law for the solvent: P = x_solvent P_solvent^*.
P = 0.92 × 23.8 = 21.896 mmHg
Compute lowering: ΔP = P^* − P = 23.8 − 21.896 = 1.904 mmHg

Frequently Asked Questions

Q: When can I use Raoult’s Law?

Raoult’s Law is most accurate for ideal solutions (similar molecules, similar intermolecular forces). For strongly non-ideal mixtures, you’d use activity coefficients (γ) instead of assuming ideality.

Q: Why is the vapor richer in one component than the liquid?

The component with the higher P^* is more volatile, so it contributes a larger fraction of the vapor pressure. That typically makes y different from x.

Q: What if my xA and xB don’t add to 1?

If you turned on Normalize, the calculator rescales them so x_A+x_B=1. If Normalize is off, it will treat the values as-is (and may warn you).

Q: Do A and B have to use the same pressure units?

Yes — both P_A^* and P_B^* must be in the same units (mmHg, kPa, atm, etc.). The outputs will be in that same unit.

Q: What does “nonvolatile solute” mean?

It means the solute contributes ~0 vapor pressure (doesn’t evaporate appreciably), so only the solvent contributes to P.

Solutions