Raoult's Law
Calculate the vapor pressure of an ideal solution using Raoult's Law. Enter pure solvent vapor pressure and moles of solute and solvent to get mole fractions, solution vapor pressure, and vapor pressure lowering with charts.
About This Calculator
The Raoult's Law Calculator helps chemistry students and professionals compute the vapor pressure of an ideal solution using the fundamental colligative property relationship. By entering the vapor pressure of the pure solvent and the amounts of solute and solvent in moles, you instantly obtain the solution vapor pressure, mole fractions of each component, and the vapor pressure lowering effect.
Raoult's Law follows the formula: p_solution = x_solvent × p°_solvent, where x_solvent is the mole fraction of the solvent (calculated as moles of solvent divided by total moles) and p°_solvent is the vapor pressure of the pure solvent. For example, if water (vapor pressure 23.8 mmHg at 25°C) is the solvent with 2 moles and you add 0.5 moles of a non-volatile solute, the mole fraction of water is 2.0/2.5 = 0.8, and the solution vapor pressure is 0.8 × 23.8 = 19.04 mmHg — a lowering of 4.76 mmHg.
Regional Notes: Raoult's Law is universally applicable and taught in chemistry curricula worldwide. In India, it is covered in Class 12 CBSE and undergraduate physical chemistry. In the US, it is included in AP Chemistry and general college chemistry courses. In the UK, it appears in A-level Chemistry and undergraduate chemical thermodynamics. The law applies equally regardless of region, though pressure units may vary (mmHg, torr, atm, kPa in different countries).
Frequently Asked Questions
What is Raoult's Law?
Raoult's Law states that the vapor pressure of an ideal solution is equal to the mole fraction of the solvent multiplied by the vapor pressure of the pure solvent. Mathematically: p_solution = x_solvent × p°_solvent, where x_solvent is the mole fraction of the solvent and p°_solvent is the vapor pressure of the pure solvent.
How do I use the Raoult's Law calculator?
Enter the vapor pressure of the pure solvent (p°), the moles of solute, and the moles of solvent. Click Calculate to get the vapor pressure of the solution, the mole fraction of solvent and solute, and the vapor pressure lowering (ΔP). The results include interactive charts comparing vapor pressures and mole fraction distribution.
What units does the calculator use for vapor pressure?
The calculator accepts any unit of pressure (mmHg, atm, kPa, torr) as long as you use the same unit consistently. The results will be displayed in the same unit you entered. The default example uses mmHg, which is commonly used for vapor pressure measurements.
What is vapor pressure lowering?
Vapor pressure lowering (ΔP) is the difference between the vapor pressure of the pure solvent and the vapor pressure of the solution. According to Raoult's Law, when a non-volatile solute is added to a solvent, the vapor pressure of the resulting solution is lower than that of the pure solvent. This is a colligative property that depends on the amount of solute particles.
Does Raoult's Law apply to all solutions?
Raoult's Law applies strictly to ideal solutions where the solute is non-volatile and the intermolecular forces between solute and solvent molecules are similar to those within the pure components. Real solutions may show positive or negative deviations from Raoult's Law, especially when solute-solvent interactions differ significantly from solute-solute or solvent-solvent interactions.
What is the mole fraction and how is it calculated?
Mole fraction (x) is the ratio of the number of moles of one component to the total number of moles in the solution. For the solvent: x_solvent = n_solvent / (n_solute + n_solvent). For the solute: x_solute = n_solute / (n_solute + n_solvent). The sum of all mole fractions in a solution always equals 1.
What are some real-world applications of Raoult's Law?
Raoult's Law is used to determine the molecular mass of unknown non-volatile solutes, to calculate boiling point elevation and freezing point depression, to design distillation processes, to understand osmotic pressure, and to predict the behavior of solutions in chemical engineering and pharmaceutical formulations.