Dalton’s Law of Partial Pressures P total =P 1 +P 2 +…. Total pressure of a mixture of gases in a container is the sum of the individual pressures (partial pressures) of each gas, as if each took up the total space alone. This is often useful when gases are collected “over water”

Collecting Gas over Water Pressure atm = Pressure (O 2 ) + Pressure (H 2 O vapor)

GIVEN: P H2 = ? P total = 94.4 kPa P H2O = 2.72 kPa WORK: P total = P H2 + P H2O 94.4 kPa = P H kPa P H2 = 91.7 kPa Dalton’s Law H 2 gas is collected over water at 22.5°C. Find the pressure of the dry gas if the atmospheric pressure is 94.4 kPa. Look up water-vapor pressure on for 22.5°C.

GIVEN: P gas = ? P total = torr P H2O = 42.2 torr WORK: P total = P gas + P H2O torr = P H torr P gas = torr A gas is collected over water at a temp of 35.0°C when the barometric pressure is torr. What is the partial pressure of the dry gas? Dalton’s Law DALTON’S LAW Look up water-vapor pressure for 35.0°C.

Graham’s Law Diffusion – Spreading of gas molecules throughout a container until evenly distributed. Effusion – Passing of gas molecules through a tiny opening in a container Smaller and lighter gas particles do this faster!

Graham’s Law Speed of diffusion/effusion – At the same temp & KE, heavier molecules move more slowly. – Ex: Which of the following gases will diffuse most rapidly? A.) N 2 B.) CO 2 C.) CH 4

V n Avogadro’s Principle Volume of a gas is directly proportional to the number of moles of gas particles present.

Equal volumes of gases contain equal numbers of moles of particles – at constant temp & pressure – true for any gas

Avogadros Law: Molar Volume of a Gas:

Practice Questions At the same temperature and pressure, 1.0 liter of CO(g) and 1.0 liter of CO2(g) have A.equal masses and the same number of molecules B. different masses and a different number of molecules C. equal volumes and the same number of molecules D. different volumes and a different number of molecules

Which rigid cylinder contains the same number of gas molecules at STP as a 2.0-liter rigid cylinder containing H 2(g) at STP? (1) 1.0-L cylinder of O 2(g) (2) 2.0-L cylinder of CH 4(g) (3) 1.5-L cylinder of NH 3(g) (4) 4.0-L cylinder of He (g

Which two samples of gas at STP contain the same total number of molecules? (1) 1 L of CO (g) and 0.5 L of N 2(g) (2) 2 L of CO (g) and 0.5 L of NH 3(g) (3) 1 L of H 2(g) and 2 L of Cl 2(g) (4) 2 L of H 2(g) and 2 L of Cl 2(g)