Vapor Pressure Calculator

Calculate vapor pressure of a substance at any temperature using the Antoine equation: log P = A - B/(C+T). Look up Antoine constants for common substances or enter your own.

💧 Phase Equilibrium📐 log P = A - B/(C+T)🧪 Chemistry
Antoine constant A
Antoine constant B
Antoine constant C
Temperature T (°C)

Water (60-150 C): A=8.07131, B=1730.63, C=233.426. Ethanol: A=8.20417, B=1642.89, C=230.300

Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Vapor Pressure Calculator—log P = A - B/(C+T)mmHg

Step-by-Step Examples

Example 1
Water at 25 C

A=8.07131, B=1730.63, C=233.426, T=25.0 C.

  • log P = 8.07131 - 1730.63/(233.426+25.0)
  • = 8.07131 - 1730.63/258.426 = 8.07131 - 6.696 = 1.375
  • P = 10^1.375 = 23.73 mmHg
✓ P(water, 25 C) = 23.73 mmHg
Example 2
Ethanol at 20 C

A=8.20417, B=1642.89, C=230.300, T=20.0 C.

  • log P = 8.20417 - 1642.89/(230.300+20.0)
  • = 8.20417 - 1642.89/250.3 = 8.20417 - 6.563 = 1.641
  • P = 10^1.641 = 43.8 mmHg
✓ P(ethanol, 20 C) = 43.8 mmHg
Example 3
Boiling Point Verification

Water should boil at 100 C (P=760 mmHg).

  • log(760) = 2.881
  • Check: A - B/(C+100) = 8.07131 - 1730.63/333.426 = 8.071-5.191 = 2.880
  • 2.880 ~ 2.881 confirms Antoine equation
✓ Antoine equation correctly predicts T_boil = 100 C

Real-World Applications

🧪
Distillation Design
Antoine equation is fundamental tool for distillation column design in chemical engineering.
🌍
Environmental Fate
Vapor pressure determines if a chemical evaporates from soil and water into atmosphere.
🛢️
Fuel Volatility
Gasoline Reid vapor pressure regulated for air quality. Higher VP = more evaporation.
🏭
Solvent Selection
Vapor pressure of solvents determines evaporation rate in coatings, adhesives, and printing inks.

Common Mistakes to Avoid

⚠️
Temperature range of Antoine constants

Antoine constants are valid only in specific T ranges. Outside this range: use different constants or Clausius-Clapeyron.

⚠️
P in mmHg for Antoine equation

Most Antoine constants give P in mmHg. Some references use kPa or bar. Check units in source.

⚠️
Antoine vs Clausius-Clapeyron

Antoine: empirical, more accurate over wide T range. Clausius-Clapeyron: theoretical, uses dHvap, valid near known data point.

Frequently Asked Questions

What is the Antoine equation? ▾
Empirical correlation: log P = A - B/(C+T). More accurate than Clausius-Clapeyron over wide temperature ranges. A, B, C are substance-specific constants.
Where do I find Antoine constants? ▾
NIST WebBook (webbook.nist.gov) has Antoine constants for thousands of substances. Perry Chemical Engineers Handbook. Yaws Chemical Properties Handbook.
What is Reid Vapor Pressure (RVP)? ▾
Vapor pressure of gasoline at 37.8 C (100 F). Regulated to limit evaporative emissions. Summer gasoline has lower RVP. EPA limits: 9-15 psi depending on region.
How does vapor pressure affect safety? ▾
High VP substances evaporate quickly: flammability and toxicity risk. Acetone (VP=56 kPa at 20 C) vaporizes much faster than water (VP=2.3 kPa). Use proper ventilation.
What temperature is boiling point? ▾
Temperature where vapor pressure equals atmospheric pressure (usually 1 atm = 760 mmHg). Antoine: solve for T given P = 760.

Related Chemistry Calculators

🌡️ Clausius-Clapeyron💧 Raoult Law🌡️ Boiling Point Elevation💨 Henry Law💨 Dalton Law

Formula Explorer connections

Interpretation: This relationship connects pressure, volume, temperature, amount or phase composition for gases and volatile mixtures. Assumption: Use absolute temperature and compatible pressure-volume units. Ideal behavior weakens at high pressure, low temperature, strong intermolecular attraction or near phase change.

Antoine Equation Calculator →Avogadro's Law Calculator →Combined Gas Law Calculator →Chemistry Formula Explorer →