3Gas Mixtures--Partial Pressure What happens when you put all three samples of gas together into one container (the same size container as each was in alone)?The gases form a homogeneous mixture.The pressure in the container increases, V and T stay the sameHow do you know what the new pressure will be?P
4Gas Mixtures--Partial Pressure Each gas in a mixture behaves independently of the other gases present.Each gas exerts its own pressure on the container.PO = pressure exerted by O2PN = pressure exerted by N2PCO = pressure exerted by CO2222
5Gas Mixtures--Partial Pressure Partial pressure: the pressure exerted by a particular gas present in a mixtureDalton's Law of Partial Pressure: The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone.Ptotal = P1 + P2 + P3 + ………
6Gas Mixtures--Partial Pressure Ptotal = PO PN + PCOSo for this example:Ptotal = 6 psi + 8 psi + 9 psi= 23 psi222
7Partial Pressure In other words, at constant T and V, Ptotal depends only on the total number of moles of gas presentPtotal is independent of the type (or types) of gases present.
8Partial Pressure-Mole Fraction When describing a mixture of gases, it is useful to know the relative amount of each type of gas.Mole fraction (X): a dimensionless number that expresses the ratio of the number of moles of one component compared to the total number of moles in a mixture.
9Mole FractionIf a gas mixture contains 5.0 mol O2 (g), 3.0 mol H2O (g), and 12.0 mol N2 (g),XO=On the exam, you must be able to calculate the mole fraction of each component of a gas mixture.nO25.0 mol= 0.25=nt20.0 mol
10Partial PressureThe partial pressure of a gas in a mixture can be found:PA = XA Ptotalwhere PA = partial pressure of gas AXA = mole fraction of gas APtotal = total pressure of mixture
11Partial Pressure Calculation A mixture of gases contains 0.51 mol N2, 0.28 mol H2, and 0.52 mol NH3. If the total pressure of the mixture is 2.35 atm, what is the partial pressure of H2?PH2 = XH2 Ptotal0.28 molXH2== 0.210.28 mol mol molPH2 = 0.21 x 2.35 = 0.50 atm
12In the lab Chemical reaction producing gas eg: NH4NO2 (s) N2(g) + H2O (l)Determine number of moles (amount) of gas collected?
13Partial PressuresWhen one collects a gas over water, there is water vapor mixed in with the gas. Ptotal = Pgas + PH2OTo find only the pressure of the desired gas, one must subtract the vapor pressure of water from the total pressure.Table p1111 shows water vapor pressure (T dep)
14Gas Mixtures--Partial Pressure What is the partial pressure of O2 in a sample of gas collected over water if the total pressure was 745 torr at 25oC?Given: Ptotal = 745 torrT = 25oCFind: PoxygenPtotal = PO2 + PH2OMust find Pwater first.
21Molecular Effusion & Diffusion What does this all mean?“Lighter” molecules will escape faster than “heavier” molecules.If you want your balloons to stay inflated longer, use N2 instead of He because N2 has a higher molar mass.
22Real Gases Real gases do not completely follow the ideal gas law. In kinetic molecular theory, the following assumptions are made:gas molecules occupy no spacegas molecules have no attraction for each other
23Real Gases Neither assumption is correct. Real gas molecules have a finite volume.Real gas molecules do attract each other.
24Real Gases The greatest deviation from ideal gas behavior occurs at: high pressurehigher density of gas moleculesMolecules are closer together so:finite volume of gas molecules more importantattraction between molecules more important
25Real GasesIn the real world, the behavior of gases only conforms to the ideal-gas equation at relatively high temperature and low pressure.Curves for 1 mol of gas
26Real Gases Low temperature Attractive forces between molecules becomes more important.Average kinetic energy decreases.Gas molecules have less energy to overcome attractive forces.
27Real GasesEven the same gas (e.g. nitrogen) will show wildly different behavior under high pressure at different temperatures.