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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.

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Presentation on theme: "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."— Presentation transcript:

1 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?

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

3 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

4 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

5 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 + ………

6 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

7 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.

8 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.

9 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

10 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

11 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

12 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?

13 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)

14 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.

15 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

16 What is happening?

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

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

19 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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21 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.

22 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

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

24 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

25 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

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

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


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