Vapor Pressure Calculator
Use the Clausius-Clapeyron equation to find a substance's vapor pressure at a new temperature, or determine its enthalpy of vaporization from two known pressure-temperature points.
🌡️ What is Vapor Pressure?
Vapor pressure is the pressure exerted by a substance's vapor when it is in equilibrium with its own liquid (or solid) phase at a given temperature. Every liquid has molecules constantly escaping into the gas phase and condensing back; vapor pressure measures how strongly that escaping tendency competes with condensation at that temperature. A liquid with high vapor pressure at room temperature, like diethyl ether, is described as volatile; one with low vapor pressure, like motor oil, is not.
Chemists and engineers use vapor pressure calculations to predict how a substance behaves under changing conditions: whether a solvent will evaporate quickly during a synthesis, at what temperature a liquid will boil under a given pressure (including reduced pressure in a vacuum distillation), how much of a volatile compound will be present in headspace above a sealed container, and how a refrigerant's pressure changes across a cooling cycle. The Clausius-Clapeyron equation is the standard tool for these calculations whenever a substance's enthalpy of vaporization is known.
A common point of confusion is boiling point versus vapor pressure. A liquid boils at the specific temperature where its vapor pressure equals the surrounding external pressure, not at some fixed universal temperature. That is why water boils at a lower temperature on a mountain (lower atmospheric pressure) than at sea level, and why a pressure cooker raises water's boiling point by increasing the pressure above it.
This calculator applies the Clausius-Clapeyron equation generally, for any pure substance, not just water. It has two modes: Find Vapor Pressure computes the pressure at a new temperature given a known enthalpy of vaporization and one reference point, and Find ΔHvap works backward from two experimentally measured pressure-temperature pairs to determine the enthalpy of vaporization itself, along with an estimate of the normal boiling point.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Diethyl Ether Vapor Pressure at Room Temperature
Diethyl ether has ΔHvap = 27.1 kJ/mol and boils at 307.75 K (34.6°C) where P = 101.325 kPa. What is its vapor pressure at 293.15 K (20°C)?
Example 2 — Isopropyl Alcohol at Room Temperature
Isopropyl alcohol (IPA) has ΔHvap = 44.0 kJ/mol and boils at 355.4 K (82.25°C) where P = 101.325 kPa. What is its vapor pressure at 293.15 K (20°C)?
Example 3 — Finding ΔHvap from Two Lab Measurements
A liquid (chloroform) was measured at P1 = 13.07 kPa at T1 = 283.15 K (10°C), and P2 = 68.86 kPa at T2 = 323.15 K (50°C). Find ΔHvap and estimate its normal boiling point.
❓ Frequently Asked Questions
🔗 Related Calculators
What is the Clausius-Clapeyron equation?
The Clausius-Clapeyron equation relates the vapor pressure of a pure substance at two different temperatures to its enthalpy of vaporization: ln(P2/P1) = -(ΔHvap/R)(1/T2 - 1/T1), where R = 8.314 J/(mol·K). It follows from the Clausius-Clapeyron relation assuming the vapor behaves as an ideal gas and the liquid's volume is negligible compared to the vapor's.
How do I calculate vapor pressure at a new temperature?
Rearrange the Clausius-Clapeyron equation for P2: P2 = P1 x e^[-(ΔHvap/R)(1/T2 - 1/T1)]. Enter a known reference pressure P1 at temperature T1, the enthalpy of vaporization ΔHvap, and your target temperature T2 in Find Vapor Pressure mode.
What is enthalpy of vaporization (ΔHvap)?
Enthalpy of vaporization is the energy required to convert one mole of a liquid into vapor at constant pressure, typically reported in kJ/mol at the normal boiling point. It is always positive because vaporization is endothermic, energy must be added to overcome intermolecular attractions.
How do I find ΔHvap from two vapor pressure measurements?
Use Find ΔHvap mode: enter two (pressure, temperature) pairs and the calculator solves ΔHvap = -R x ln(P2/P1) / (1/T2 - 1/T1). This is the standard method for extracting ΔHvap from lab data without needing a reference table.
Does this calculator work for any liquid, not just water?
Yes. This calculator is substance-agnostic: you supply the enthalpy of vaporization and a reference point yourself, so it works for any pure liquid, ethanol, benzene, diethyl ether, ammonia, or anything else, as long as you have or can look up its ΔHvap. For water specifically, the separate Vapor Pressure of Water Calculator skips the ΔHvap lookup entirely.
Why must temperature be in Kelvin?
The Clausius-Clapeyron equation involves 1/T terms, which only make physical sense on an absolute temperature scale where zero means zero molecular kinetic energy. Using Celsius would divide by temperatures that can be zero or negative, producing meaningless results. Always convert Celsius to Kelvin by adding 273.15 before entering a temperature.
How accurate is the Clausius-Clapeyron equation?
It is exact only if ΔHvap is constant with temperature, the vapor is an ideal gas, and the liquid's molar volume is negligible next to the vapor's. All three are approximations. Accuracy is best over a narrow temperature range near the reference point and degrades over wide ranges, particularly as the substance approaches its critical point.
What does the estimated normal boiling point in Find ΔHvap mode mean?
Once ΔHvap is known from two data points, the same equation can be solved for the temperature at which vapor pressure equals 1 standard atmosphere (101.325 kPa), which by definition is the normal boiling point. This calculator reports that estimate automatically alongside ΔHvap.
Can I use this for sublimation (solid to gas) instead of vaporization?
The same mathematical form applies to sublimation if you substitute the enthalpy of sublimation (ΔHsub) for ΔHvap, since sublimation follows the same Clausius-Clapeyron relation. Enter ΔHsub as the ΔHvap input; the rest of the calculation is unchanged.
What units does R use in this calculator?
R = 8.314 J/(mol·K), the standard gas constant. The enthalpy input is taken in kJ/mol and converted internally to J/mol before use, so you can enter ΔHvap directly in the units most references publish it in.