Vapor Pressure Calculator

The liquid eVaporates, the gaseous molecules formed will escape in the air. But, if the liquid is kept in a closed container, the gaseous molecules will not escape in the air but will remain above the liquid. Those eVaporated particles will create a pressure above the liquid, which is then known as the Vapor pressure.units of the Vapor pressure is the torr. 1 torr =1 mm Hg.

There are two Simple ways to find the Vapor pressure, that is the Clausius-Clapeyron equation and Raoult’s law.

Clausius-Clapeyron Equation:The Clausius-Clapeyron equation is used to find the Vapor pressure at any other temperature if the Vapor pressure, the enthalpy of Vaporization are known. If the temperature increases, then the Vapor 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 and 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 big 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
• ΔH is the enthalpy of Vaporization.

Raoult’s Law:

A solution is created when a solid gets dissolved into the liquid. The Vapor pressure formed from this solution is lowered by the addition of the solute. Here comes the Vapor pressure formula using Raoult’s law,.normal atmospheric pressure is 1 atm (760 mmHg or 101.325 kPa).

P_solution = (X_solvent )( P^o _solvent )

• Where X_solvent = mole fraction of the solvent in the solution
• P_solution = Vapor pressure of the solution
• P^o _solvent = Vapor pressure of the solvent

Procedure to calculate Vapor pressure of a solution by using the equation Clausius - Clapeyron equation is explained as follows.

• Step 1: Enter the values of initial and final temperature in kelvin/centigrade/fahrenheit.
• Step 2: Enter the values of initial and final pressure in pascals etc..
• Step 3: Press the calculate button, finally we got the molar enthalpy of a Vaporisation

Procedure to calculate Vapor pressure of a solution with Raoult’s law, is as follows

• Step 1: Enter the value of mole fraction.
• Step 2: Enter the value Vapor pressure of the solvent.
• Step 3: Click the calculate button, to get the Vapor pressure of the solution.

Example 1.: The Vapor pressure of an aqueous solution is found to be 23.80 mmHg at 25 °C. What is the mole fraction of solute in this solution? The Vapor pressure of water is 25.756 mm Hg at 25 °C ?

Solution:

Given that,

Vapor pressure of the solution P solution = 23.80mmHg

Vapor pressure of the solvent P°solvent = 25.756 mm Hg

By using roult’s law formula

23.80 = X solvent * 25.756

X solvent = 0.92405

X solvent + X solute = 1

X solute = 1- X solvent

= 1 - 0.92405 = 0.07595

The mole fraction of the solute in the solution is 0.07595

Example 2.: The Vapor pressure of an aqueous solution is found to be 23.80 mmHg at 25 °C. What is the mole fraction of solute in this solution? The Vapor pressure of water is 25.756 mm Hg at 25 °C ?

Solution:

Given that,

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

Initial pressure = 20 torr

Initial temperature = 14.7°C

Final temperature = 52.8°C

Convert °C to K

TK = °C + 273.15

T1 = 14.7 °C + 273.15

T1 = 287.85 K

T2 = 52.8 °C + 273.15

T2 = 325.95 K

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

ln(20/P₂) = (47.2/0.008314) [1/325.95 - 1/287.85]

ln(20/P₂) = 5677(-4.06 x 10-4) = -2.305

P₂/20 = 10.02

P₂ = 200.4

Therefore, the Vapor pressure of 1-propanol at 52.8°C is 200.4 torr

1. What is meant by Vapor pressure?

Vapor pressure is a measure of the tendency of a material to change into a gases state or Vapor state and its increases with temperature.

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2. What are the factors affecting the Vapor Pressure?

There are three common factors that influence the Vapor pressure are surface area, intermolecular force and temperature. The Vapor pressure of a molecule is differ at different temperatures.

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3. What are the Units of Vapor Pressure?

Vapor pressure is measured in units of atmospheres(atm),millimetres of mercury(mmHg) or kilopascals(KPa).

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4. Which instrument is used to measure Vapor pressure?

A manometer( U shaped glass tube with marks in the background) is a pressure measuring instrument.usually limited to measuring pressures near to the atmosphere.

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5. What are the characteristics of Vapor pressure?

The important characteristics of Vapor pressure are given below,

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

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6. What is the formula for Vapor pressure?

To find Vapor pressure we can use two equations in chemistry, The Clausius-Clapeyron equation gives the Vapor 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 * Xsolvent.