Upon completion of this topic, learners will be able to:

• describe the relationship between partial pressure and the total pressure as described in Dalton’s Law of partial pressure.

Thus far we have considered mainly pure gases—those that consist of only one substance in the gaseous state. How do we deal with mixtures of two or more different gases? While studying the properties of air, John Dalton made an important observation: The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone. The pressure exerted by a particular component of a mixture of gases is called the partial pressure of that component. Dalton’s observation is known as Dalton’s law of partial pressures.

Let P_t be the total pressure of a mixture of gases and P₁, P₂, P₃ and so forth be the partial pressures of the individual gases, we can write Dalton’s law of partial pressures as:

P_t = P₁ + P₂ + P₃ + ....

This equation implies that each gas behaves independently of the others, as we can see by the following analysis.

Collecting a gas over water

A useful application of the law of partial pressures arises when you collect gases over water (a method used for gases that do not dissolve appreciably in water). How a gas, produced by chemical reaction in the flask, is collected by leading it to an inverted tube, where it displaces water. As gas bubbles through the water, the gas picks up molecules of water vapor that mix with it. The partial pressure of water vapor in the gas mixture in the collection tube depends only on the temperature. This partial pressure of water vapor is called the vapor pressure of water.

The following example shows how to find the partial pressure and then the mass of the collected gas.