Vapour Pressure Calculator

The vapor pressure is the pressure that is generated by the evaporation of liquids or solids. It occurs at equilibrium when the molecules are vaporizing and condensing at the same rate at a particular pressure. The pressure is directly related to the kinetic energy of a molecule. Vapour pressure changes with the temperature of the surroundings and the liquid nature.

The two simple ways to calculate the vapor pressure is the Clausius-Clapeyron equation, Raoult's law. The important details about those equations are mentioned here.

The Clausius-Clapeyron equation is used to find the vapour pressure at any other temperature if the vapor pressure, the enthalpy of vaporization are known. If the temperature increases, then the vapour pressure increases steadily.

The Clapeyron equation is dP/dT = H / (T * ΔV)

Where,

• dP/dT is a derivative of pressure with respect to temperature
• H is the specific latent heat (the thermal energy absorbed or released during a phase transition)
• T is the temperature.
• ΔV is the change of the specific volume

The Clausius-Clapeyron equation describes the relation between the vapor pressure the liquid temperature. The phase transition between the liquid and gas is vaporization or between solid and gas is the sublimation. If there is a huge difference between the specific volume of the molecule gas phase and the condensed phase, then we use the following formula.

ln(P₁/P₂) = ΔH/R * (1/T₂ - 1/T₁)

Where,

• T₁ is the initial temperature
• P₁ is the initial pressure
• T₂ is the final temperature
• P₂ is the final pressureΔH is the molarity enthalpy of vaporization
• R is the universal gas constant = 8.3145 J/mol*K

Enthalpy of vaporization is the energy needed for a phase change like converting liquid into gas.

Example:

Question: The vapor pressure of 1-propanol is 10 torr at 14.7°C. Calculate the vapor pressure at 52.8°C

Answer:

Heat of vaporization of 1-propanol = 47.2 kJ/mol

Initial pressure = 10 torr

Initial temperature = 14.7°C

Final temperature = 52.8°C

The Clausius-Clapeyron equation is ln(P₁/P₂) = ΔH/R * (1/T₂ - 1/T₁)

ln(10/P₂) = (47.2/0.008314) [1/325.95 - 1/287.85]

ln(10/P₂) = 5677(-4.06 x 10-4) = -2.305

P₂/10 = 10.02

P₂ = 100.2

Therefore, the vapor pressure of 1-propanol at 52.8°C is 100.2 torr

Raoult's law states that the solvent vapour pressure in a solution is equal to the vapor pressure of the pure solvent times the mole fraction of the solution.

Psolution = Psolvent x Xsolvent

Where,

• Psolution is the vapor pressure of the solution
• Psolvent is the vapor pressure of pure solvent
• Xsolvent is the mole fraction of the solvent

Example:

Question: The vapor pressure of an aqueous solution is 23.80 mmHg at 25°C. What is the mole fraction of solute in the solution? The vapour pressure of water is 25.756 mmHg at 25°C.

Answer:

Given that

Vapour pressure of solution = 23.80 mmHg

Vapor pressure of solvent = 25.756 mmHg

Using Raoult's law Psolution = Psolvent x Xsolvent

23.80 = 25.756 x Xsolvent

Xsolvent = 0.92405

Xsolute = 1 - 0.92405

= 0.07595

Therefore, the mole fraction of the solute is 0.07595.

Chemistrycalc.Com has got plenty of calculators designed to help you on the concepts such as molecular weight, entropy, atomic mass, partial pressure etc. all in one place.

1. What is meant by vapour pressure?

The vapour pressure is defined as the pressure caused due to the evaporation of liquids or solids. It is formed due to the thermodynamic equilibrium state with liquids or solids at the given temperature in a closed system.

2. How to calculate vapor pressure?

We can use these two equations to find the vapour pressure in chemistry. The Clausius-Clapeyron equation gives the vapour pressure at a temperature i.e ln(P₁/P₂) = ΔH/R * (1/T₂ - 1/T₁). Second one is Raoult's law i.e Psolution = Psolvent x Xsolvent. Substitute the values in these formulas to get the answer.

3. What are the characteristics of vapour pressure?

The important characteristics of vapour pressure are listed here:

• A pure liquid experiences a higher vapour pressure against the liquid's solution.
• The vapour pressure increases with an increase in temperature.
• It is inversely proportional to the forces of the attraction between the existing molecules of a liquid.

4. Define Raoult's law?

Raoult's law states that the partial pressure is proportional to the mole fraction of the solute component. The vapor pressure of a solution is equal to the pressure of the pure solvent times the mole fraction.

Psolution = Psolvent x Xsolvent.