Dalton's Law

Dalton's Law, named for the same John Dalton who gave us the atomic theory, is unlike the previous laws in that it is not about a relationship between two different properties of a gas.

In this case, the law is a simple statement that pressures are additive (can be added).

Dalton's Law is commonly stated as:

The sum of the partial pressures of individual gases in a mixture of gases 

is equal to the total pressure. 

In English, that says that if you know the pressure caused by each individual gas in a mixture and you add them up, you will get the total pressure. This seems painfully simple and obvious on the surface but there are some interesting ideas lurking beneath the surface.

First, this law implies that the pressure of one gas is totally independent of any other gases present. This means that if you have a container that holds oxygen gas at 1.50 atm and you then add some nitrogen to the same container, the pressure of the oxygen (the force that the oxygen exerts on the walls) will be unchanged. If this seems okay to you, you can just go on, or you can look into it further.

Dalton's Law also implies that the identity of a gas is irrelevant to the pressure. A mole of gas in a container will create a certain pressure, no matter what that gas is. In other words, there is no way to tell what gas is causing a certain pressure (or portion of the pressure).

There are three types of problems that are associated with Dalton's Law. We will call them proportional pressures, connected containers, and collection of a gas over water.