Dalton's law expresses the fact that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture. This equality arises from the fact that in an ideal gas, the molecules are so far apart that they do not interact with each other. Most actual real-world gases come very close to this ideal. For example, given an ideal gas mixture of nitrogen (N), hydrogen (H) and  ammonia (NH):
where:
* = total pressure of the gas mixture
* = partial pressure of nitrogen (N)
* = partial pressure of hydrogen (H)
* = partial pressure of ammonia (NH)