The key difference between Raoult law and Dalton law is that Raoult law deals with the vapour pressure of solids or liquids, whereas Dalton law deals with the partial pressure of non-reacting gases.
Raoult law and Dalton law are very important laws in chemistry that explain the partial pressures of gaseous states. Raoult law describes the behaviour of the partial pressure of vapour of a solution when changing the solute concentration. In contrast, Dalton law describes the behaviour of non-reacting gases in the same vessel.
CONTENTS
1. Overview and Key Difference
2. What is Raoult Law
3. What is Dalton Law
4. Side by Side Comparison – Raoult Law vs Dalton Law in Tabular Form
6. Summary
What is Raoult Law?
Raoult law states that the vapour pressure of a solvent above a solution is equal to the vapour pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present in the solution. We can give this relationship mathematically as follows:
Psolution=Xsolvent.Posolvent
Where Psoltuion is the vapour pressure of the solution, Xsolvent is the mole fraction of the solvent and Posolvent is the vapour pressure of the pure solvent. The law was developed by the French chemist, François-Marie Raoult in 1880. He discovered that when adding a solute to a solution, the vapour pressure of the solution gradually decreases. However, this observation was dependent on two variables: mole fraction of the dissolved solute and vapour pressure of the pure solvent.
Figure 01: Vapor Pressure of a Binary Solution that Follows Raoult’s Law
At a given pressure for a particular solid or liquid, there is a pressure at which the vapour of the substance is in equilibrium with the substance in solid or liquid form. At that temperature, we name the pressure above the substance as the vapour pressure. Furthermore, at this equilibrium, the rate of the evaporation of solid or liquid substance is equal to the vapour that condenses back to the solid or liquid form. Thus, this is the basic theory behind Raoult law. However, Raoult law works for ideal solutions. But it also works well with solvents in a very dilute state. For real substances (not ideal substances), the decrease in vapour pressure is practically greater than the value we calculate from the Raoult law.
What is Dalton Law?
Dalton law states that the total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of each gas. The law was developed by John Dalton in 1802. We can give this law mathematically as follows:
Ptotal=Pi
Where Ptotal is the total pressure of gas mixture while Pi is the partial pressure of each individual gas.
Figure 02: Dalton Law
For example, if we have a non-reacting gas mixture with three components in it, we can write the relationship as follows:
Ptotal = P1+P2+P3
What is the Difference Between Raoult Law and Dalton Law?
Raoult law and Dalton law are very important laws in chemistry that explain the partials pressures of gaseous states. The key difference between Raoult law and Dalton law is that the Raoult law deals with the vapour pressure of solids or liquids, whereas the Dalton law deals with the partial pressure of non-reacting gases. That is; the Raoult law states that the vapour pressure of a solvent above a solution is equal to the vapour pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present in the solution. Meanwhile, the Dalton law states that the total pressure of a mixture of non-reacting gases equals the sum of the partial pressures of each gas. The mathematical expression for Raoult law is Psolution=Xsolvent.Posolvent while the mathematical expression for Dalton law is Ptotal=Pi.
Summary – Raoult Law vs Dalton Law
Raoult law and Dalton law are very important laws in chemistry that explain the partials pressures of gaseous states. However, the key difference between Raoult law and Dalton law is that the Raoult law deals with the vapour pressure of solids or liquids, whereas the Dalton law deals with the partial pressure of non-reacting gases.
Reference:
1. Helmenstine, Anne Marie. “Raoult’s Law Definition in Chemistry.” ThoughtCo, Jan. 9, 2019, Available here.
Image Courtesy:
1. “Raoultov zakon” (Public Domain) via Commons Wikimedia
2. “Schematic Depicting Dalton’s Law” By BlyumJ – Own work (CC BY-SA 4.0) via Commons Wikimedia