# Dalton’s Law of Partial Pressure

## Dalton’s Law of Partial Pressure

Dalton’s Law of Partial Pressure was observed by John Dalton in 1801. This law is related to ideal gas laws. The pressure exerted by an individual gas is a mixture is known as its partial pressure. It is the sum of the partial pressures of individual gases is equal to the total pressure exerted by the mixture of non-reactive gases.

Dalton’s law of partial pressure says that the pressure exerted by a mixture of several gases equals the sum of the pressures exerted by each gas occupying the same volume as that of the mixture.

In kinetic theory, we assume that the pressure exerted by a gas on the walls of a container is due to the collision of the molecules with the walls. The total force on the wall is the sum of the forces exerted by the individual molecules. Suppose there are N₁ molecules of gas 1, N₂ molecules of gas 2 etc. in the mixture.

Thus, the force on a wall of surface area A,

F = Force by N₁ molecules of gas 1 + Force by N₂ molecules of gas 2 + … = F₁ + F₂ + …

Thus, the pressure, P = (F₁/ A) + (F₂/ A) + …

If the first gas alone is kept in the container, its N₁ molecules will exert a force F₁ on the wall. If the pressure in this case is P₁,

Then, P₁ = F₁/ A, Similarly for other gases.

Thus,

P = P₁ + P₂ + P₃ + P₄ + …

Where,

P = Total Pressure exerted by the mixture of gases.

P₁, P₂, P₃, … are the Partial Pressures of the gases.

Generally, gases are collected over water and therefore, are moist in nature. Hence, while calculating the pressure of a dry gas, we reduce the vapour pressure of water from the total pressure of the moist gas. The pressure exerted by the saturated water vapour is referred to as the aqueous tension. Aqueous tensions are different at different temperatures.