Do Now & Announcements Turn in Lab from yesterday Take out notes from yesterday—you can staple it to today’s packet as these are the notes for the entire unit Today: Pressure, Dalton & Graham’s Law

Pressure Barometer- used to measure atmospheric (air) pressure The higher the altitude the __________ the atmospheric pressure and the _________ the height of the mercury in the barometer lower Whatever the height is, that is your pressure

Measuring Pressure of a Gas Manometer- measures the pressure of an enclosed sample Can be open or closed Closed Manometer

Pressure Gas pressure is less than atmospheric pressure when the height of the liquid in the manometer is higher on the _______________. Therefore you will ________________ the height and the atmospheric pressure. Open Manometer -h +h left side of the U subtract Gas pressure is greater than atmospheric pressure when the height of the liquid in the manometer is higher on the _______________. Therefore you will ________________ the height and the atmospheric pressure. right side of the U add

Examples 1. If the atmospheric pressure is 757.8mmHg, what is the pressure of the gas in each of the following manometers?

Dalton’s Law 1) the total pressure exerted by a mixture of gases is the sum of the individual pressures of each gas 2) each individual gas behaves as if it were independent of the others. P total = P 1 + P 2 + P 3 + …

List all variables Write the formula of the appropriate Gas Law Plug & Chug Final Answer is… (check your work!) Known: Unknown: Examples 1. Two gases such as oxygen and nitrogen are present in a flask at the following pressures. When combined, what is the pressure of the flask? P Nitrogen =250. mm Hg, P Oxygen =300. mm Hg P N =250. mm Hg, P O =300. mm Hg P Total P total =P 1 + P 2 … P total =P N + P O P total = mm Hg

Examples 2. Neon gas has a pressure of 1.49atm in its container. When added to a container holding helium gas the total pressure is 2.34atm. What is the pressure of the helium gas? List all variables Write the formula of the appropriate Gas Law Plug & Chug Final Answer is… (check your work!) Known: Unknown: P Ne =1.49 atm P total =2.34 atm P He P total =P 1 + P 2 … P total =P Ne + P He 2.34 = P He 0.85 atm

Dalton’s Law Water displacement of gas: Gases given off from the rxn travel through the tube and into the adjacent container for collection Gas is impure and contains some water vapor P total = P gas + P H2O Value depends on temperature

Dalton’s Law Examples Oxygen gas from the decomposition of potassium chlorate was collected by water displacement. The barometric pressure and the temperature during the experiment were 731.0torr and 20.0˚C respectively. What was the partial pressure of the oxygen collected? The vapor pressure of water at 20°C is 17.5 torr List all variables Write the formula of the appropriate Gas Law Plug & Chug Final Answer is… (check your work!) Known: Unknown: P total =731 torr P H2O =17.5 torr P oxygen P total =P gas + P H2O P total =P oxygen + P H2O 731=P oxygen torr

Some hydrogen gas was collected over water at 20.0 ˚C. The partial pressure of hydrogen is torr. What is the barometric pressure of the gas? The vapor pressure of water at 20°C is 17.5 torr List all variables Write the formula of the appropriate Gas Law Plug & Chug Final Answer is… (check your work!) Known: Unknown: P Hydrogen =742.5 torr P H2O =17.5 torr P total P total =P gas + P H2O P total =P hydrogen + P H2O P total = torr

Summary Table: Dalton’s Law P total = P 1 + P 2 + P 3 + … P total = P gas + P H2O

Graham’s Law of Diffusion Under ideal conditions, the rates at which different gases diffuse (spread out) are inversely proportional to their molar masses. The rate of effusion and diffusion depend upon the velocity (speed) of the gas molecules Speed is dependent upon molar mass In other words, larger gas molecules will move slower than smaller gas molecules.

Graham’s Law

Graham’s Law Examples 1. Compare the rates of diffusion of H 2 and O 2 gases at the same temperature and pressure. H 2 will diffuse more quickly than O 2 because it has a smaller molar mass (H 2 = 2.02 g/mol, O 2 = g/mol) 2. Rank the following gases from slowest to fastest rate of diffusion: H 2, CO 2, Ne, H 2 S. 3. Compare the rates of effusion of nitrogen and helium at the same temperature and pressure. 4. Compare the rate of effusion of carbon dioxide with hydrochloric acid at the same temperature and pressure.

Summary Table: Graham’s Law Higher Molar Mass  Slower Speed (Smaller Molar Mass  Faster Speed) inversely relationship

Gas Laws Summary Table Add to bottom of chart: STP= Standard Temperature & Pressure 273 K &1 atm