Gases I. What is a gas? There are 3 common states of matter: Solid, Liquid, Gas Solid - Has a fixed shape and volume - Not compressible (molecules tightly packed in a rigid, organized manner; no room for them to compress) Liquid - Has a fixed volume, but NO fixed shape (takes the shape of the container) - Not compressible (molecules are in closed contract but have sufficient room to slide past each other) Gas - NO fixed shape and NO fixed volume (takes the shape and volume of the container) - Can be compressed (all molecules are widely separated)

Example: What volume does 28.0g of nitrogen occupy as a solid? A liquid? A gas? 28.0 g solid N 2 Liquid N 2 Gaseous N mL 34.6 mL mL II. Pressure Weight: Force of gravity exerted on an object - It is proportional to the mass of the object Pressure: Force exerted on a given area - Usually measured in Newtons per square meter (N/m 2 ) (1N ~force of gravity acting upon a 100g mass)

Pressure of a gas The pressure of a gas is the result of all the molecules of a gas moving rapidly and colliding with the walls of its container The pressure of a gas is often expressed in pascals (Pa) 1N/m 2 = 1Pa

Normal air pressure is kPa at sea level (1kPa = 1000 Pa) Another unit of pressure is given by the height of a column of mercury (mm of Hg ) (torr)

III Partial Pressure What happens when you mix two gases together? There is so much space between the gas molecules that they can easily share the same space. Each gas behaves independent of the other and each contributes to the total pressure. The partial pressure of a gas in a mixture is defined as the pressure exerted by the gas if it were there by itself.

Dalton’s Law of Partial Pressure The total pressure of a gas mixture is the sum of the partial pressures of all the gases in the mixture. P total = P a + P b + P c + … + P z Examples: mol air in 1L at 25°C has a pressure of 12.4 kPa and mol water vapour in 1L has a pressure of 2.70 kPa. What is the pressure of a mixture containing mol air mol water vapour in 1L at 25°C? From Dalton’s Law of Partial Pressure we know: P total = P air + P water P total = 12.4 kPa + 2.7kPa P total = 15.1 kPa

A bulb contains mol H 2 (g), mol (g) N 2 and mol He (g). The total pressure in the bulb is kPa. What is the partial pressure of each gas? We know: a) P total = P H2 + P N2 + P He b) The pressure exerted by a gas is directly proportional to the number of moles present. therefore P total Total moles P H2 moles H 2 = Total moles = = mol P N2 moles N 2 = P He moles He = kPa mol P H mol = P H2 = 16.0 kPa

104.0 kPa mol P N mol = P N2 = 48.0 kPa Similarly: kPa mol P He mol = P He = 40.0 kPa Now we check our answers: P total = P H2 + P N2 + P He P total = P total = kPa