Raoult's Law

The Raoult's Law calculator computes vapor pressure where solvent molecules can escape as a gas from a liquid solvent based on the mole fraction of solvent and the initial vapor pressure of solvent.

INSTRUCTIONS: Choose units and enter the following:

• (P_i^o) Initial Vapor Pressure
• (n_a) Moles in Solute A
• (n_b) Moles in Solvent B

Partial Pressure (P_i): The calculator returns the pressure in atmospheres (atm).  However, this can be automatically changed to compatible units via the pull-down menu. 

The Math / Science

The Raoult's Law^[1] equation calculates the vapor pressure at which the solvent molecules are able to escape as from the liquid phase in to the gas phase, computed by a french chemist Francois-Marie Raoult(1830-1901).  The evaporation of the solvent molecules depends on the increase of entropy^[3], if solute is added to the solvent, the entropy is increased more causing less solvent molecules to escape in to the gas phase. 

The Raoult's Law is calculated by:

         `P_i =n_a/(n_a+n_b) * P_i^0`

where: 

• P_i is the partial pressure at which the solvent escapes in to the gas phase in units (atm)
• n_a Moles in Solute A
• n_b Moles in Solvent B
• X_a is the mole fraction of solvent A 
• P_i⁰ is the initial vapor pressure in units of (atm) 

Note:  This formula uses the mole fraction

  `x_a =n_a/(n_a+n_b)`

where:

• x_a = mole fraction
• n_a = moles in solute A
• n_b = moles in solvent B

Related Topics

• Partial Pressure
• Mole fraction
• Henry's Law
• Entropy 

Supplement Material

• UC Davis: Raoult's Law
• Raoults Law and Vapor Pressure- Chemistry Tutorial

References

[1]https://en.wikipedia.org/wiki/Raoult%27s_law

[2]Whitten, et al. 10^th Edition. Pp.518

[3]http://webs.morningside.edu/slaven/Physics/entropy/entropy7.html#note