1. Mixtures of gases Section 10.1

2. Gases in the Atmosphere 300 years ago, air was thought to be one compound. Antoine Lavoisier (1743-1794) discovered that air was actually made up of at least two gases (one that supported combustion and one that did not) His experiments led to the law of conservation of mass for chemical reactions.

3. When Dalton was conducting his studies, which led him to the atomic theory of matter, he also included studies of the behaviour of gases. These led him to propose, in 1803, what is now called Dalton's law of partial pressures: ◦ The total pressure of a mixture of non- reacting gases is equal to the sum of the partial pressures of the individual gases Dalton’s Law of Partial Pressures

4. This law can be expressed as: P total = P 1 + P 2 + P 3 +... + P n Where P total is the total pressure of the mixture and P 1, P 2, and P 3 are the partial pressures each gas has in the mixture. Dalton’s Law of Partial Pressures

6. Furthermore, the pressure exerted by one of the gases in the mixture (its partial pressure) is equal to the mole fraction of that gas multiplied by the total pressure: moles of gas x X P total = P x total moles Where P total is the total pressure of the mixture and P x is the partial pressure of gas x. Dalton’s Law of Partial Pressures

7. The law is based on two concepts from the KMT. 1. The pressure of a gas is caused by the collisions with the walls of the container. 2. Gas molecules act independently of each other.

8. Vapour Pressure Dalton found that when water was placed in a closed container, some of the water evaporated to form water vapour, and that the water vapour exerted a pressure on the container. This pressure is known as the vapour pressure of water.

9. Collecting Gases over Water When gases are collected over water, the vapour pressure of water has to be taken into account The vapour pressure of water is specific to its temperature, and is usually given to you (pg. 464 Table 3)

10. P dry gas = P total – P water vapour

11. Homework Read 460 - 465 “Dalton’s Law of Partial Pressures” worksheet Page 463 # 5-7 Page 465 # 8-10