Vapor Pressure

Clausius-Clapeyron equation

Molar Enthalpy
of Vaporization
x J/mol

Initial Pressure:  


Final Pressure:  


Initial Temperature:  


Final Temperature:  


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Vapor Pressure Calculator

Vapour Pressure Calculator

The vapor pressure of a substance explains its behaviour with the temperature change. Additionally, it helps in understanding the phenomenon of evaporation and volatility.

On this online vapor pressure calculator page, you will learn what vapour pressure is, the significance of vapor pressure, and various fascinating facts about vapor pressure.

Vapour Pressure Definition

Vapour pressure is the pressure exerted by a vapor when it is at equilibrium with its solid or liquid states or both, at a given temperature.

Equilibrium is a thermodynamic state of a substance where all of its possible forms (solid, liquid, gas/vapor) exist at the same time.

Understanding Vapour Pressure

Putting it simply, the molecules of a substance always exert force on the atmosphere. They try to break their molecular bonds and become free. This is a similar concept we come across in the density of a substance too.

When heat is given to a substance, the atoms gain energy and gradually try to come out of their bonds. Thus, when sufficient heat energy is given to a substance, it is a common phenomenon that it changes its state into the next freest state possible.

i.e, when heated to a sufficient temperature, a solid turns into liquid, and a liquid converts into vapor. The same happens in vapor pressure too.

The vapor pressure is the pressure exerted by the surface molecules of a solid/liquid in the atmosphere. They keep pushing the molecules of the atmosphere with this pressure.

When temperature increases, they attain more energy (their kinetic energy increases) and then the push becomes stronger. This is why vapor pressure increases with an increase in temperature.

The following graph shows how steadily the vapor pressure of various liquids increases with temperature.


Note: Vapour pressure decreases with an increase in the intermolecular forces of the liquid.

Significance of Vapour Pressure

Vapour pressure plays an important role in many daily life activities and industry applications.

  • Vapor pressure is one of the factors that determine the temperature of distillation of liquids. When a liquid contains two or more immiscible liquids, irrespective of their concentration, the temperature of distillation depends on their vapor pressure.
  • Vapour pressure is also the property that governs the volatility of a substance. A liquid that has more vapor pressure is said to be more volatile and evaporates faster.
  • Vapour pressure is an important property of a substance. It plays a significant role in classifying a substance as hazardous.
  • Threshold Limit Value is a term used in assessing the safety of workers working with chemicals as a part of their daily job. Vapour pressure is an important indicator in measuring TLV and designing safety gear for such risk-prone workers.
  • Vapour pressure also plays a critical role in calculating the time taken for evaporation of oil spills (in the case of volatile oils/chemicals).
  • The boiling point of a liquid is the temperature at which vapor pressure equals atmospheric pressure.

Do you know?

Vapour pressure plays a significant role in the dynamics of cooking especially when it comes to deep-frying or cooking with fat. Triglycerides have extremely low vapor pressures, so evaporation does not occur.

Some of the commonly used liquids and their vapor pressures at 20 degrees centigrade are tabulated below:

Methyl alcohol - 6.10.13 atm
Toluene - 9.20.03 atm
Water - 0.02 atm

Vapour pressure calculation

You can calculate vapour pressure using the following formulae:

By Using Clausius-Clapeyron equation

Vapor pressure is calculated by using the Clausius-Clapeyron equation as shown here:

ln(P1/P2)=ΔHvap R (1/T2-1/T1)


P1 and P2 are the vapor pressures at two temperatures T1 and T2.

ΔHvap is the Enthalpy (heat) of Vaporization and R is the gas constant (8.3145 J mol-1 K-1).

By Using Raoult’s law:

The vapor pressure of liquid mixtures is calculated by using Raoult’s Law.

According to Raoult’s Law, the vapor pressure of a solution is directly proportional to the mole fraction of the solute.

If P is the vapor pressure of the solvent, x is its mole-fraction and Pv is the vapor pressure of the pure solvent, then:

According to Raoult’s law,

P xA

P = Pv * xA

Vapour pressure of a mixture of liquids

The vapor pressure of a liquid mixture containing two liquids A and B is given by:

P total = PA0* xA + PB0* xB

PA and PB are the partial vapor pressures of liquids A and B respectively
xA and xB are the mole fractions of liquids A and B respectively

Do you know?

We know the boiling point of water is 100 degrees centigrade at normal pressure 1 bar. But would not it be surprising to know that water vapor can exist below this temperature too? Yes. When atmospheric pressure is reduced, the vapor pressure of water increases and water turns into vapor at a much lesser temperature.

For instance, at 0.5 bar, the boiling point of water becomes 81°C.

Similarly, if the atmospheric pressure is further lowered to 0.25 bar, the boiling point of water reaches 65°C.

Thus, vapor pressure is an important concept to understand many fascinating aspects of liquids.

FAQs about Vapour Pressure

How is vapor pressure measured?
Vapour pressure is measured by a vacuum pump and a glass apparatus.

What are the units of vapor pressure?
The units of vapor pressure are atmosphere (atm), millimeters of mercury (mmHg), or kilopascals (kPa).

Which has more vapor pressure – a single liquid or a solution?
A single liquid has more vapor pressure than a solution with two or more liquids.

What is the shape of the plot of the vapor pressure of a solvent in a solution?
The relation between the mole fraction of a solvent and the vapor pressure of the solution is linear. Hence, the plot would be shaped in a straight line.

What is the relation between the surface area of the liquid and its vapor pressure?
The more the surface area of the liquid, the higher the chance for molecules to exert pressure on the atmosphere. Hence, vapor pressure increases with an increase in the surface area of the liquid.

What happens to the vapor pressure of a liquid when we add a non-volatile solute to it?
On the addition of a non-volatile solute, the surface area of the liquid in contact with the atmosphere decreases. Hence its vapor pressure decreases.

Online vapour pressure calculator

Now that you are equipped with the vapor pressure concepts, make your vapor pressure calculations simpler with our online vapor pressure calculator. Don’t forget to explore our other online calculators and the fascinating aspects we uncovered for you on those pages. If you need this vapour pressure calculator widget free, please reach us at Keep calculating with us, for free every single time!

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