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Temperature–Vapor Pressure & Raoult’s Law

Relate two equilibrium vapor pressures and temperatures with constant vaporization enthalpy, or calculate an ideal solution’s solvent partial pressure.

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

In the Raoult-law section, solvent mole fraction 0.4 and pure-solvent vapor pressure 1,000 Pa give a partial pressure of 400 Pa.

ln(P1/P2) = (ΔHvap/R)(1/T2 − 1/T1); psolvent = xsolvent × psolvent,pure

Worked example

Enter these known values and leave the other values blank.

Mole fraction of the solvent
.4
Vapor pressure of the solvent
1000 Pa
Solvent partial pressure above solution
400 Pa

Assumptions and limitations

  • The two sections are independent. Clausius–Clapeyron assumes constant positive molar vaporization enthalpy, ideal vapor and negligible liquid volume over the range.
  • The retained Clausius–Clapeyron coefficient is R = 8.3145 J/(mol·K). Absolute pressures must be positive and temperatures above absolute zero. Equal temperatures give equal pressures, but cannot identify a unique enthalpy.
  • Raoult’s law gives the solvent partial pressure for an ideal solution. It equals total vapor pressure only when other components have negligible vapor pressure.
  • Solvent mole fraction is between zero and one. At zero fraction this component’s partial pressure is zero; it does not establish the total pressure of the mixture.

Common questions

What assumptions matter for this calculation?

The two sections are independent. Clausius–Clapeyron assumes constant positive molar vaporization enthalpy, ideal vapor and negligible liquid volume over the range.

References

Bookify uses strictly positive vapor pressures inside logarithms, permits equal temperatures, and bounds ideal-solution mole fraction including zero partial pressure. Mercury pressure columns use conventional density and standard gravity, with the exact international inch. Pounds-force per square inch use the exact international pound and inch with standard gravity, replacing the rounded captured factor. One technical atmosphere is exactly one kilogram-force per square centimeter under standard gravity.

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