Vapor Pressure Calculator at a Given Temperature

Calculate a liquid’s vapor pressure at a given temperature with the Antoine equation: enter the temperature and the substance’s Antoine coefficients to get its estimated vapor pressure. The result is only meaningful when the coefficients match the substance, temperature range, and pressure units you intend to use.

Formula

The calculator uses the Antoine equation, an empirical relationship commonly used to estimate a pure substance’s saturation vapor pressure over a specified temperature range:

log₁₀(P) = A − B / (T + C)

• P is the saturation vapor pressure, in the pressure unit specified by the coefficient source. This calculator assumes mmHg.
• T is temperature in °C.
• A, B, and C are Antoine coefficients for the substance and the stated temperature interval.

The calculator converts the result from mmHg to kPa using 1 mmHg ≈ 0.133322368 kPa. This conversion changes units, not the underlying estimate. Antoine coefficients are not universal constants: a different coefficient set may use a different pressure unit or temperature scale. Check the source before entering values.

Worked Example

Using the default values—T = 25 °C, A = 8.07131, B = 1730.63, and C = 233.426—calculate:

1. Substitute into the equation: log₁₀(P) = 8.07131 − 1730.63 / (25 + 233.426).
2. Evaluate the denominator and fraction: 25 + 233.426 = 258.426; 1730.63 / 258.426 ≈ 6.697.
3. Therefore log₁₀(P) ≈ 1.374, and P = 10^1.374 ≈ 23.76 mmHg.
4. Convert units: 23.76 × 0.133322368 ≈ 3.17 kPa.

The displayed results are approximately 23.76 mmHg and 3.17 kPa. Small differences in the last digit can result from rounding intermediate values; the calculator evaluates the expression before rounding its displayed outputs.

How to Choose the Right Coefficients

Find an Antoine coefficient set for the exact chemical, then verify its temperature range, temperature scale, and pressure unit. Data tables often publish more than one set for a substance because one set may fit a limited interval better than a single equation across a broad range. Do not combine A, B, or C values from different rows or sources.

• Use coefficients for the correct substance and, where relevant, the specified chemical form or mixture component.
• Keep the input temperature within the range stated for that coefficient set. Extrapolating beyond it can give poor estimates.
• Confirm that the source’s pressure unit is mmHg before using this calculator’s pressure output. If it specifies another unit, convert the result or use a calculator configured for that unit.
• Check whether the source expects °C or another temperature scale. The default set here is for temperature in °C.

The default coefficients are a familiar water coefficient set intended for a limited temperature interval around ordinary liquid-water conditions. They are not suitable at every temperature, and the calculator does not validate a coefficient set or warn when an input lies outside its published range.

What Vapor Pressure Means

In a closed system at equilibrium, molecules leave a liquid and enter the vapor while others return to the liquid. The vapor pressure is the pressure exerted by that vapor at the specified temperature when liquid and vapor are in equilibrium. For a pure liquid, saturation vapor pressure rises as temperature rises because more molecules have enough energy to escape the liquid.

Vapor pressure is not the same as the total pressure above a liquid. If other gases are present, their partial pressures contribute to total pressure as well. Nor is it simply a measure of how quickly a liquid evaporates: evaporation rate also depends on factors such as airflow, exposed surface area, and humidity.

When to Use the Estimate and Its Limits

Use the calculation when you need an approximate saturation pressure for a pure substance and have suitable Antoine coefficients. It can help compare how vapor pressure changes with temperature or convert a tabulated relationship into a value at an intermediate temperature. The output is an equilibrium property estimate, not a direct measurement of a particular open container.

Real mixtures may behave differently from a pure liquid. A solution’s vapor pressure depends on its composition and interactions between components; calculating it may require mixture-specific data or an appropriate thermodynamic model. The Antoine equation is also empirical, so its accuracy is limited by the data fit and the coefficient set’s stated range. For demanding work, consult authoritative property data and consider measurement uncertainty rather than treating the displayed digits as guaranteed precision.

Common Calculation Mistakes

• Using the wrong temperature scale: entering a Fahrenheit value as though it were °C produces an incorrect result. Convert first: °C = (°F − 32) × 5/9.
• Mixing coefficient sets: coefficients must come together as one published set for the same substance and range.
• Assuming the output unit: the Antoine equation returns pressure in the unit associated with its coefficients. The calculator’s first output assumes that unit is mmHg.
• Extrapolating: a mathematically calculable result outside the data range is not necessarily physically reliable.
• Confusing vapor pressure with boiling point: boiling occurs when a liquid’s saturation vapor pressure equals the surrounding pressure. The same liquid therefore boils at different temperatures under different external pressures.

Frequently Asked Questions

How do I calculate vapor pressure from temperature?
Use a vapor-pressure relation such as the Antoine equation, with temperature and matching substance-specific coefficients. Convert the answer from the coefficient set’s pressure unit if needed.

Does vapor pressure increase with temperature?
For a given pure liquid over its ordinary liquid range, its saturation vapor pressure increases as temperature rises. Use data valid for the temperature being evaluated.

What units does this calculator use?
Temperature is entered in °C. The first result is in mmHg, and a second result is converted to kPa. The coefficients must be compatible with those assumptions.

Can I use the Antoine equation for a mixture?
Not by simply entering coefficients for one component. Mixture vapor pressure depends on composition and non-ideal interactions, so additional data or a suitable mixture model may be needed.

Why does my result differ from a table?
Possible causes include a different coefficient set, an input outside its fitted range, different units, or rounding. Compare the source’s temperature interval and pressure units before comparing values.

Related

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in t | Temperature | °C | 25
in a | Antoine A | - | 8.07131
in b | Antoine B | - | 1730.63
in c | Antoine C | - | 233.426
out p | Vapor pressure | mmHg | 10^(a - b / (t + c)) | 2
out kpa | Vapor pressure | kPa | p * 0.133322368 | 2