Ch. 12.1 – Gases III. One More Law Dalton’s Law Pages 408 – 410

Ptotal = P1 + P2 + ...Pn A. Dalton’s Law (of partial pressures) The total pressure of a mixture of gases equals the sum of the partial pressures of the individual gases Ptotal = P1 + P2 + ...Pn

A. Dalton’s Law A sample of air has a total pressure of 1.00 atm. The mixture contains only CO2, N2, and O2. If PCO2 = 0.12 atm and PN2 = 0.70 atm, what is the partial pressure of O2? GIVEN: PO2 = ? Ptotal = 1.00 atm PCO2 = 0.12 atm PN2 = 0.70 atm WORK: Ptotal = PCO2 + PN2 + PO2 1.00 atm = 0.12 atm + .70 atm + PO2 PO2 = 0.18 atm

What you are trying to find A. Dalton’s Law (over water) When H2 gas is collected by water displacement, the gas in the collection bottle is actually a mixture of H2 and water vapor Water exerts its own pressure known as water-vapor pressure So, to determine total pressure of gas and water vapor inside a container, make total pressure inside bottle = atmosphere and use this formula: What you are trying to find Look up on table Always known Ptotal = Pgas + PH2O

A. Dalton’s Law Hydrogen gas is collected over water at 22.5°C. Find the pressure of the dry gas if the atmospheric pressure is 94.4 kPa. The total pressure in the collection bottle is equal to atmospheric pressure and is a mixture of H2 and water vapor. GIVEN: PH2 = ? Ptotal = 94.4 kPa PH2O = 20.4 mm Hg = 2.72 kPa WORK: Ptotal = PH2 + PH2O 94.4 kPa = PH2 + 2.72 kPa PH2 = 91.7 kPa Look up water-vapor pressure for 22.5°C, convert to kPa Sig Figs: Round to lowest number of decimal places.

A. Dalton’s Law GIVEN: Pgas = ? Ptotal = 742.0 torr PH2O = 42.2 torr A gas is collected over water at a temp of 35.0°C when the barometric pressure is 742.0 torr. What is the partial pressure of the dry gas? The total pressure in the collection bottle is equal to barometric pressure and is a mixture of the “gas” and water vapor. GIVEN: Pgas = ? Ptotal = 742.0 torr PH2O = 42.2 torr WORK: Ptotal = Pgas + PH2O 742.0 torr = Pgas + 42.2 torr Pgas = 699.8 torr Look up water-vapor pressure for 35.0°C. Sig Figs: Round to least number of decimal places.