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Gas Mixtures--Partial Pressure So far: pure gases Many gases are actually mixtures of two or more gases: –air: O 2, N 2, H 2 O, etc How do mixtures of gases behave?

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Gas Mixtures--Partial Pressure P= 8 psi N 2 (g) P= 6 psi O 2 (g) P= 9 psi CO 2 (g)

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Gas 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 same –How do you know what the new pressure will be? P

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Gas Mixtures--Partial Pressure Each gas in a mixture behaves independently of the other gases present. –Each gas exerts its own pressure on the container. P O = pressure exerted by O 2 P N = pressure exerted by N 2 P CO = pressure exerted by CO

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Gas Mixtures--Partial Pressure Partial pressure: the pressure exerted by a particular gas present in a mixture Dalton'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. P total = P 1 + P 2 + P 3 + ………

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Gas Mixtures--Partial Pressure P total = P O + P N + P CO So for this example: P total = 6 psi + 8 psi + 9 psi = 23 psi 2 2 2

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Partial Pressure In other words, at constant T and V, –P total depends only on the total number of moles of gas present –P total is independent of the type (or types) of gases present.

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Partial 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.

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Mole Fraction If a gas mixture contains 5.0 mol O 2 (g), 3.0 mol H 2 O (g), and 12.0 mol N 2 (g), XO=XO= On the exam, you must be able to calculate the mole fraction of each component of a gas mixture. nO2nO2 ntnt = 5.0 mol 20.0 mol = 0.25

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Partial Pressure The partial pressure of a gas in a mixture can be found: P A = X A P total whereP A = partial pressure of gas A X A = mole fraction of gas A P total = total pressure of mixture

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Partial Pressure Calculation A mixture of gases contains 0.51 mol N 2, 0.28 mol H 2, and 0.52 mol NH 3. If the total pressure of the mixture is 2.35 atm, what is the partial pressure of H 2 ? P H 2 = X H 2 P total XH2=XH2= 0.28 mol 0.28 mol mol mol = 0.21 P H 2 = 0.21 x 2.35 = 0.50 atm

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In the lab Chemical reaction producing gas eg: NH 4 NO 2 (s) N 2 (g) + H 2 O (l) Determine number of moles (amount) of gas collected?

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Partial Pressures When one collects a gas over water, there is water vapor mixed in with the gas. P total = P gas + P H 2 O To 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)

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Gas Mixtures--Partial Pressure What is the partial pressure of O 2 in a sample of gas collected over water if the total pressure was 745 torr at 25 o C? Given: P total = 745 torr T = 25 o C Find:P oxygen P total = P O2 + P H2O Must find P water first.

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Partial Pressure P total = P oxygen + P water To find P water, look in Appendix (p 1111): At 25 C, P water = torr So: 745 torr = P O torr P O2 = 745 torr torr = 721 torr

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What is happening?

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Effusion Effusion is the escape of gas molecules through a tiny hole into an evacuated space.

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Diffusion Diffusion is the spread of one substance throughout a space or throughout a second substance.

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Molecular Effusion & Diffusion The rate of effusion (r) of a gas is inversely proportional to the square root of its molar mass, M. r A = 1 MAMA

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Molecular 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 N 2 instead of He because N 2 has a higher molar mass.

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Real Gases Real gases do not completely follow the ideal gas law. In kinetic molecular theory, the following assumptions are made: –gas molecules occupy no space –gas molecules have no attraction for each other

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Real Gases Neither assumption is correct. –Real gas molecules have a finite volume. –Real gas molecules do attract each other.

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Real Gases The greatest deviation from ideal gas behavior occurs at: high pressure higher density of gas molecules –Molecules are closer together so: »finite volume of gas molecules more important »attraction between molecules more important

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Real Gases In 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

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Real Gases –Low temperature Attractive forces between molecules becomes more important. –Average kinetic energy decreases. –Gas molecules have less energy to overcome attractive forces.

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Real Gases Even the same gas (e.g. nitrogen) will show wildly different behavior under high pressure at different temperatures.

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