What this calculator does
Vapor pressure (Clausius–Clapeyron) works out finds vapor pressure at a new temperature from a known point and enthalpy of vaporization. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
The inputs explained
| Field | What to enter |
|---|---|
| Known vapor pressure P₁ (atm) | A number, measured in atm. Starts at 1. |
| Known temperature T₁ (K) | A number, measured in K. Starts at 373.15. |
| Enthalpy of vaporization ΔHvap (J/mol) | A number, measured in J/mol. Starts at 40700. |
| New temperature T₂ (K) | A number, measured in K. Starts at 298.15. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with known vapor pressure p₁
Every other input is held at the calculator’s starting values while known vapor pressure p₁ varies. Select any row to load that scenario into the calculator.
| Known vapor pressure P₁ (atm) | Vapor pressure at T₂ | Reference pressure at T₁ | Note |
|---|---|---|---|
| 0.5 | 0.0184397 atm | 0.5000 atm | Assumes ΔHvap is constant over the temperature range and vapor behaves ideally. |
| 0.75 | 0.02765955 atm | 0.7500 atm | Assumes ΔHvap is constant over the temperature range and vapor behaves ideally. |
| 1 | 0.0368794 atm | 1.000 atm | Assumes ΔHvap is constant over the temperature range and vapor behaves ideally. |
| 1.5 | 0.0553191 atm | 1.500 atm | Assumes ΔHvap is constant over the temperature range and vapor behaves ideally. |
| 2 | 0.07375879 atm | 2.000 atm | Assumes ΔHvap is constant over the temperature range and vapor behaves ideally. |
| 3 | 0.11063819 atm | 3.000 atm | Assumes ΔHvap is constant over the temperature range and vapor behaves ideally. |