# What is the vapor pressure?

Finds a liquid’s vapor pressure at a new temperature with the Clausius–Clapeyron equation, the temperature for a vapor pressure, or the enthalpy of vaporization from two points, or the vapor pressure from Antoine constants.

- Page: https://www.acalculator.org/chemistry/vapor-pressure-calculator
- JSON spec: https://www.acalculator.org/chemistry/vapor-pressure-calculator.json
- Version: 35023be2474e

## Default answer

Example with the default inputs (Find Vapor pressure, ΔHvap (kJ/mol) 31.3, Temperature T₁ 56.5 °C, Vapor pressure P₁ 101.3, Temperature T₂ 25 °C, Pressure unit kPa): P₂ = 30.3128 kPa.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| find | Find | P₂, T₂ or ΔHvap with the Clausius–Clapeyron equation, or the pressure from Antoine constants. |
| dh | ΔHvap (kJ/mol) | The enthalpy of vaporization in kilojoules per mole, such as 40.7 for water at its boiling point. |
| t1 | Temperature T₁ | A temperature where you know the vapor pressure, such as the normal boiling point. |
| p1 | Vapor pressure P₁ | The vapor pressure at T₁: 101.325 kPa (1 atm) at the normal boiling point. |
| t2 | Temperature T₂ | The other temperature. |
| p2 | Vapor pressure P₂ | The vapor pressure at T₂, in the same unit as P₁. |
| aa | Antoine A | NIST’s A for the substance, with P in bar and T in kelvins. |
| ab | Antoine B | NIST’s B for the substance, in kelvins. |
| ac | Antoine C | NIST’s C for the substance, in kelvins. |
| t | Temperature T | The temperature, inside the range NIST gives for the constants. |
| pu | Pressure unit | The unit of the pressures typed and shown. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Answer | The vapor pressure, the temperature, or ΔHvap worked out. |
| pressure | Vapor pressure | The vapor pressure in the unit chosen. |
| pBar | Vapor pressure (bar) | The vapor pressure in bars. |
| tK | Temperature T₂ (K) | The temperature worked out, in kelvins. |
| tC | Temperature T₂ (°C) | The temperature worked out, in degrees Celsius. |
| dhKj | ΔHvap (kJ/mol) | The enthalpy of vaporization in kilojoules per mole. |

## Method

ln(P₂ ÷ P₁) = (ΔHvap ÷ R)(1 ÷ T₁ − 1 ÷ T₂), so P₂ = P₁ × e^((ΔHvap ÷ R)(1 ÷ T₁ − 1 ÷ T₂)), 1 ÷ T₂ = 1 ÷ T₁ − (R ÷ ΔHvap) ln(P₂ ÷ P₁) and ΔHvap = R ln(P₂ ÷ P₁) ÷ (1 ÷ T₁ − 1 ÷ T₂); Antoine: log₁₀(P ÷ bar) = A − B ÷ (T + C).

## Assumptions

- ΔHvap does not change with temperature between T₁ and T₂, and the vapor behaves as an ideal gas.
- Antoine constants hold only in the temperature range their source gives; the page does not check it.

## Worked examples

1. find = h, t1 = 307.15, p1 = 10, t2 = 371.95, p2 = 100, pu = kPa gives dhKj = 33.7528, result = ΔHvap = 33.7528 kJ/mol. Source: OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.7 Estimating Enthalpy of Vaporization: 33.8 kJ/mol.
2. find = h, t1 = 293.15, p1 = 5.95, t2 = 336.65, p2 = 53.3, pu = kPa gives dhKj = 41.3582. Source: OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.7 Check Your Learning: 41,360 J/mol.
3. find = t2, dh = 30.8, t1 = 353.25, p1 = 101.3, p2 = 83.4, pu = kPa gives tK = 346.819, tC = 73.6694. Source: OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.8 Estimating Temperature (or Vapor Pressure): 346.9 K (73.8 °C) from 353.3 K.
4. find = p2, dh = 31.3, t1 = 329.65, p1 = 101.3, t2 = 298.15, pu = kPa gives pressure = 30.3128, result = P₂ = 30.3128 kPa. Source: OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.8 Check Your Learning: 30.1 kPa.
5. find = antoine, aa = 4.6543, ab = 1,435.264, ac = -64.848, t = 298.15, pu = kPa gives pBar = 0.0317938, pressure = 3.17938, result = P = 3.17938 kPa (0.0317938 bar). Source: NIST Chemistry WebBook, SRD 69, Water: Antoine equation parameters (log₁₀(P) = A − B/(T + C), P in bar, T in K; Stull 1947, 255.9 to 373 K: A = 4.6543, B = 1435.264, C = −64.848), https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=4&Type=ANTOINE&Plot=on.

## FAQ

### How do I calculate vapor pressure at a different temperature?

Use the two-point Clausius–Clapeyron equation, ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂), with temperatures in kelvins. Acetone boils at 56.5 °C (329.65 K) at 101.3 kPa with ΔHvap = 31.3 kJ/mol; at 25.0 °C, P₂ = 101.3 × e^(−1.20652) = 30.31 kPa.

### How do I find the enthalpy of vaporization from two vapor pressures?

Rearrange: ΔHvap = R ln(P₂/P₁) ÷ (1/T₁ − 1/T₂). Isooctane has 10.0 kPa at 34.0 °C and 100.0 kPa at 98.8 °C, so ΔHvap = 8.314 × ln 10 ÷ (1/307.15 − 1/371.95) = 33.75 kJ/mol (OpenStax: 33.8 kJ/mol).

### How do I find the boiling point at a lower pressure?

A liquid boils when its vapor pressure equals the pressure above it. Solve for T₂ with P₂ = the new pressure: benzene boils at 80.1 °C at 101.3 kPa, and at 83.4 kPa (Denver) at about 346.8 K, or 73.7 °C.

### What is the Antoine equation?

An empirical fit of vapor pressure: log₁₀ P = A − B ÷ (T + C). The NIST Chemistry WebBook lists A, B and C for many substances with P in bar and T in kelvins, each for a temperature range. For water from 255.9 to 373 K (Stull, 1947): A = 4.6543, B = 1435.264, C = −64.848, which gives 0.0318 bar (3.18 kPa) at 25 °C.

### Why does my answer differ from the textbook’s?

Textbooks round temperatures and R along the way. OpenStax converts 80.1 °C to 353.3 K and gets 346.9 K for benzene; with 353.25 K the page gives 346.8 K. For acetone it gives 30.1 kPa where the unrounded equation gives 30.3 kPa.

### Does the pressure unit matter?

No, as long as P₁ and P₂ share it: the equation uses their ratio. The page offers kPa, atm, mmHg, torr, bar, Pa and psi, and shows the Antoine result in bar as well.

## Sources

- OpenStax, Chemistry 2e, §10.3 Phase Transitions: the Clausius–Clapeyron equation (ln P = −ΔHvap/RT + ln A and ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂), T in kelvins, any matching pressure units), Example 10.7 (isooctane: 33.8 kJ/mol) and its Check Your Learning (ethanol: 41,360 J/mol), Example 10.8 (benzene at 83.4 kPa: 346.9 K, 73.8 °C) and its Check Your Learning (acetone at 25.0 °C: 30.1 kPa). https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions (retrieved 2026-10-19)
- NIST Chemistry WebBook, SRD 69, Water, phase change data: Antoine equation parameters, log₁₀(P) = A − (B/(T + C)) with P in bar and T in K; 255.9 to 373 K (Stull, 1947): A = 4.6543, B = 1435.264, C = −64.848. https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=4&Type=ANTOINE&Plot=on (retrieved 2026-10-19)
- NIST, CODATA value: molar gas constant R = 8.314 462 618… J/(mol·K), exact (N_A × k). https://physics.nist.gov/cgi-bin/cuu/Value?r (retrieved 2026-10-19)
- NIST, Special Publication 811, Appendix B.8: standard atmosphere = 101 325 Pa, torr = 101 325/760 Pa, conventional millimeter of mercury = 133.322 387 415 Pa, bar = 100 000 Pa. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8 (retrieved 2026-10-19)