Vapor Pressure Calculator: Estimate Substance Volatility

Vapor Pressure Calculator

A vapor pressure calculator estimates the pressure a liquid's vapor exerts when the liquid and vapor are in balance at a given temperature. This tool uses the Antoine equation to calculate that pressure, in millimeters of mercury (mmHg), for eight common substances: water, methanol, ethanol, acetone, benzene, toluene, chloroform, and diethyl ether.

What is vapor pressure?

Every liquid has molecules moving at different speeds. Some of the fastest ones at the surface break free and turn into gas, even without boiling. This escaping gas builds up pressure above the liquid, and eventually the rate of molecules leaving the liquid equals the rate of molecules returning to it. The pressure at that balance point is the vapor pressure.

A liquid with high vapor pressure evaporates quickly, because many of its molecules escape easily. Acetone and diethyl ether are examples: they evaporate fast at room temperature. Water has a lower vapor pressure at room temperature and evaporates more slowly.

Vapor pressure rises as temperature rises. Warmer molecules move faster, so more of them have enough energy to escape the liquid. This relationship is not a straight line; vapor pressure increases faster and faster as temperature climbs.

Vapor pressure formula: the Antoine equation

The calculator uses the Antoine equation, a formula fitted to real measured data for each substance:

$\log_{10}(P) = A - \frac{B}{C + T}$

• P is the vapor pressure, in mmHg
• T is the temperature, in °C
• A, B, and C are constants unique to each substance, found by fitting the equation to lab measurements

Each substance's constants only work correctly inside a specific temperature range. Outside that range, the equation can give a wrong answer, so the calculator will not compute one.

Antoine constants used by this calculator

How to use the vapor pressure calculator

1. Choose a substance from the dropdown menu.
2. Enter a temperature in °C. It must fall inside the substance's valid range, shown below the input.
3. Read the vapor pressure result in mmHg.
4. Check the formula panel to see the exact numbers used in the calculation.
5. Press "Copy" to copy the result.

If the temperature is outside the valid range, the calculator shows an error message instead of a result, because the Antoine equation is unreliable there.

How to calculate vapor pressure: worked example

Find the vapor pressure of water at 25°C.

Step 1: Get the constants for water. A = 8.07131, B = 1730.63, C = 233.426

Step 2: Put them into the equation.

$\log_{10}(P) = 8.07131 - \frac{1730.63}{233.426 + 25} = 8.07131 - \frac{1730.63}{258.426} = 1.3745$

Step 3: Undo the logarithm.

$P = 10^{1.3745} \approx 23.7 \text{ mmHg}$

Water's vapor pressure at 25°C is about 23.7 mmHg. That is much lower than normal atmospheric pressure (760 mmHg at sea level), which is why liquid water does not boil at room temperature.

As a check on the formula: at 100°C, the same equation gives a vapor pressure of about 760 mmHg for water, matching water's known boiling point at sea level.

Vapor pressure of common substances at 25°C

Acetone and diethyl ether have much higher vapor pressures than water at the same temperature. That is why they evaporate faster and feel colder on skin: fast evaporation carries heat away quickly.

Reading the result

• A higher vapor pressure means the substance evaporates more easily at that temperature.
• A lower vapor pressure means the substance stays liquid more readily.
• A liquid boils at the temperature where its vapor pressure equals the surrounding pressure. At sea level, that surrounding pressure is about 760 mmHg (1 atmosphere).

Where vapor pressure matters

Vapor pressure guides many real tasks. Chemical engineers use it to design distillation columns, which separate liquids by boiling off the more volatile one first. It affects how storage tanks for solvents and fuels are built, since a liquid with high vapor pressure can build up dangerous pressure in a sealed container. Environmental scientists use it to predict whether a pollutant will evaporate from water into air. Pharmacists use it to judge how a liquid medicine will behave in its packaging. Laboratory workers use it to choose safe conditions for vacuum distillation and rotary evaporation, and to judge how to store volatile chemicals safely.

Other ways to find vapor pressure

The Antoine equation is one of several methods:

• The Clausius-Clapeyron equation is a more fundamental formula linking vapor pressure to temperature and the heat needed to vaporize the substance.
• The Wagner equation covers wider temperature ranges but needs more input constants.
• Direct measurement in a lab, using devices such as an isoteniscope, gives the most reliable single data point.
• Group contribution methods estimate vapor pressure from a molecule's structure when no experimental data exists.

History

Vapor pressure as a formal idea grew out of 19th-century work on heat and gases. Benoît Paul Émile Clapeyron and Rudolf Clausius developed the Clausius-Clapeyron equation in the 1830s, linking vapor pressure to temperature and latent heat. In 1888, French engineer Louis Charles Antoine published the simpler correlation that bears his name. It sacrifices some theoretical grounding for ease of use, which is why it remains common in textbooks and calculators today.

Frequently asked questions

What is vapor pressure in simple terms? It is the pressure of the gas that forms above a liquid when the liquid and its gas are in balance, at a set temperature. A substance with high vapor pressure evaporates easily.

How does temperature affect vapor pressure? Vapor pressure rises as temperature rises, and it rises faster and faster rather than in a straight line. Warmer molecules move faster and more of them can escape the liquid surface.

What is the difference between vapor pressure and atmospheric pressure? Vapor pressure comes from one substance's own molecules escaping into gas form. Atmospheric pressure is the combined pressure of all the gases in the air above a location. A liquid boils once its vapor pressure reaches the atmospheric pressure around it.

Why does vapor pressure matter for distillation? Distillation separates liquids by their different vapor pressures. The substance with the higher vapor pressure evaporates first and can be collected separately.

How accurate is the Antoine equation? Inside a substance's listed temperature range, it typically matches measured vapor pressure within a few percent. Outside that range, its accuracy drops, which is why this calculator blocks calculations outside each substance's valid range.

Does this calculator work for mixtures? No. It calculates vapor pressure for pure substances only. Mixtures follow Raoult's law, where each component's partial vapor pressure depends on its share of the mixture, and non-ideal mixtures need further correction factors.

References

1. Poling, B. E., Prausnitz, J. M., & O'Connell, J. P. (2001). The Properties of Gases and Liquids (5th ed.). McGraw-Hill.
2. Smith, J. M., Van Ness, H. C., & Abbott, M. M. (2017). Introduction to Chemical Engineering Thermodynamics (8th ed.). McGraw-Hill Education.
3. Antoine, C. (1888). "Tensions des vapeurs: nouvelle relation entre les tensions et les températures." Comptes Rendus des Séances de l'Académie des Sciences, 107, 681-684.
4. NIST Chemistry WebBook, SRD 69. National Institute of Standards and Technology.