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Vapor pressure

Solve vapor pressure relationships quickly using core chemistry formulas.

Formula shown7 inputs definedUpdated Nov 2025By Automated Chemistry GeneratorFree, no sign-up

Quick Summary

Use this when you need:

Estimate how vapor pressure changes with temperature for distillation
Account for solute mole fraction when predicting solution vapor pressure
Teach the link between ΔH_vap and the slope of ln P vs 1/T

You’ll need:Final temperature, Initial pressure, Initial temperature, Molar enthalpy of vaporization, and 3 more

You’ll get:Final pressure

How this is calculated

Formula:

ln(P2/P1) = -ΔH_vap / R · (1/T2 - 1/T1); P_solution = χ_solvent × P°

In plain language:

The calculator uses the Clausius–Clapeyron relation to relate two vapor pressures at different temperatures and Raoult's law (P_solution = χ_solvent P°) to include mole fraction effects when a solvent is diluted.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

What these terms mean

Final pressure

Final pressure used in the calculation.

Final temperature

Final temperature used in the calculation.

Initial pressure

Initial pressure used in the calculation.

Initial temperature

Initial temperature used in the calculation.

Molar enthalpy of vaporization

Molar enthalpy of vaporization used in the calculation.

Mole fraction of the solvent

Mole fraction of the solvent used in the calculation.

Vapor pressure of the solution

Vapor pressure of the solution used in the calculation.

Vapor pressure of the solvent

Vapor pressure of the solvent used in the calculation.

When to use this calculator

Estimate how vapor pressure changes with temperature for distillation
Account for solute mole fraction when predicting solution vapor pressure
Teach the link between ΔH_vap and the slope of ln P vs 1/T

Last updated: November 19, 2025

Vapor pressure calculator explained

Vapor pressure changes exponentially with temperature and drops when a nonvolatile solute is present. This calculator combines the Clausius-Clapeyron relationship,

\\ln\\left(\\frac{P_2}{P_1}\\right) = -\\frac{\\Delta H_{\\text{vap}}}{R}\\left(\\frac{1}{T_2} - \\frac{1}{T_1}\\right),

with Raoult's law $P = \\chi_{\\text{solvent}} P^\\circ$ so you can move between {initial_pressure}, {final_pressure}, temperatures, enthalpy of vaporization {molar_enthalpy_vaporization}, and solution vapor pressure {vapor_pressure_solution}.

Use it to estimate distillation pressures, predict boiling-point elevation, or demonstrate how mole fraction controls vapor pressure lowering.

How the conversion works

Temperature dependence: Solve the Clausius-Clapeyron expression for $P_2$ given $P_1$ , $T_1$ , $T_2$ , and $\\Delta H_{\\text{vap}}$ .
Solution effect: Apply Raoult's law with the solvent mole fraction {mole_fraction} and pure-solvent vapor pressure {vapor_pressure_solvent}.

The calculator keeps $R = 8.314$ J mol $^{-1}$ K $^{-1}$ , so temperatures must be in kelvin.

Units and conversions

Quantity	Units	Notes
$P$	kPa, bar, or mmHg	Keep $P_1$ and $P_2$ in the same unit.
$T$	K	Convert deg C to K by adding 273.15.
$\\Delta H_{\\text{vap}}$	J/mol	Use tabulated latent heats.
$\\chi_{\\text{solvent}}$	dimensionless	Equals 1 for pure solvent.

Worked examples

Water vapor pressure at 60 deg C
Known: $P_1 = 101.3\\ \\text{kPa}$ at $T_1 = 373\\ \\text{K}$ , $\\Delta H_{\\text{vap}} = 40.7\\ \\text{kJ}\\,\\text{mol}^{-1}$ , find $P_2$ at $T_2 = 333\\ \\text{K}$ .

\\ln\\left(\\frac{P_2}{101.3}\\right) = -\\frac{40{,}700}{8.314}\\left(\\frac{1}{333} - \\frac{1}{373}\\right)

P_2 = 19.9\\ \\text{kPa}

The calculator reproduces this familiar value for water at 60 deg C.

Vapor pressure lowering by sugar
A solution has $\\chi_{\\text{solvent}} = 0.98$ and the pure-solvent vapor pressure at the same temperature is $P^\\circ = 19.9\\ \\text{kPa}$ .

P = 0.98 \\times 19.9 = 19.5\\ \\text{kPa}

Even nonvolatile solutes reduce vapor pressure, which raises the boiling point by $\\Delta T_b = \\frac{K_b m}{\\chi_{\\text{solvent}}}$ .

Tips and pitfalls

Always convert temperatures to kelvin before taking reciprocals.
Use absolute pressures; gauge pressures shift the reference point.
For large temperature spans, integrate a temperature-dependent $\\Delta H_{\\text{vap}}$ or use Antoine coefficients from data tables.
Raoult's law assumes ideal behavior; strong solute-solvent interactions require activity coefficients.

References and further reading

Frequently Asked Questions

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Calculator info

Version:: v1.0.0
Last reviewed:: Nov 15, 2025
Sensitivity:: low
References:: 1

Last updated: November 19, 2025

Vapor pressure calculator explained

Vapor pressure changes exponentially with temperature and drops when a nonvolatile solute is present. This calculator combines the Clausius-Clapeyron relationship,

\\ln\\left(\\frac{P_2}{P_1}\\right) = -\\frac{\\Delta H_{\\text{vap}}}{R}\\left(\\frac{1}{T_2} - \\frac{1}{T_1}\\right),

Use it to estimate distillation pressures, predict boiling-point elevation, or demonstrate how mole fraction controls vapor pressure lowering.

How the conversion works

Temperature dependence: Solve the Clausius-Clapeyron expression for $P_2$ given $P_1$ , $T_1$ , $T_2$ , and $\\Delta H_{\\text{vap}}$ .
Solution effect: Apply Raoult's law with the solvent mole fraction {mole_fraction} and pure-solvent vapor pressure {vapor_pressure_solvent}.

The calculator keeps $R = 8.314$ J mol $^{-1}$ K $^{-1}$ , so temperatures must be in kelvin.

Units and conversions

Quantity	Units	Notes
$P$	kPa, bar, or mmHg	Keep $P_1$ and $P_2$ in the same unit.
$T$	K	Convert deg C to K by adding 273.15.
$\\Delta H_{\\text{vap}}$	J/mol	Use tabulated latent heats.
$\\chi_{\\text{solvent}}$	dimensionless	Equals 1 for pure solvent.

Worked examples

Water vapor pressure at 60 deg C
Known: $P_1 = 101.3\\ \\text{kPa}$ at $T_1 = 373\\ \\text{K}$ , $\\Delta H_{\\text{vap}} = 40.7\\ \\text{kJ}\\,\\text{mol}^{-1}$ , find $P_2$ at $T_2 = 333\\ \\text{K}$ .

\\ln\\left(\\frac{P_2}{101.3}\\right) = -\\frac{40{,}700}{8.314}\\left(\\frac{1}{333} - \\frac{1}{373}\\right)

P_2 = 19.9\\ \\text{kPa}

The calculator reproduces this familiar value for water at 60 deg C.

Vapor pressure lowering by sugar
A solution has $\\chi_{\\text{solvent}} = 0.98$ and the pure-solvent vapor pressure at the same temperature is $P^\\circ = 19.9\\ \\text{kPa}$ .

P = 0.98 \\times 19.9 = 19.5\\ \\text{kPa}

Even nonvolatile solutes reduce vapor pressure, which raises the boiling point by $\\Delta T_b = \\frac{K_b m}{\\chi_{\\text{solvent}}}$ .

Tips and pitfalls

Always convert temperatures to kelvin before taking reciprocals.
Use absolute pressures; gauge pressures shift the reference point.
For large temperature spans, integrate a temperature-dependent $\\Delta H_{\\text{vap}}$ or use Antoine coefficients from data tables.
Raoult's law assumes ideal behavior; strong solute-solvent interactions require activity coefficients.

Quick Summary

Calculator Tool

How this is calculated

What these terms mean

Final pressure

Final temperature

Initial pressure

Initial temperature

Molar enthalpy of vaporization

Mole fraction of the solvent

Vapor pressure of the solution

Vapor pressure of the solvent

When to use this calculator

Vapor pressure calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Osmotic pressure

pKa

Beer-Lambert law

Molarity

Mole

Neutralization

Was this calculator helpful?

Found an error?

Calculator info

Vapor pressure calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Osmotic pressure

pKa

Beer-Lambert law

Molarity

Mole

Neutralization

Was this calculator helpful?

Found an error?

Calculator info