Gas Mixtures--Partial Pressure P= 6 psi P= 8 psi P= 9 psi O2 (g) N2 (g) CO2 (g)

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

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. PO = pressure exerted by O2 PN = pressure exerted by N2 PCO = pressure exerted by CO2 2 2 2

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. Ptotal = P1 + P2 + P3 + ………

Gas Mixtures--Partial Pressure Ptotal = PO + PN + PCO So for this example: Ptotal = 6 psi + 8 psi + 9 psi = 23 psi 2 2 2

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

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.

Mole Fraction If 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. nO2 5.0 mol = 0.25 = nt 20.0 mol

Partial Pressure The partial pressure of a gas in a mixture can be found: PA = XA Ptotal where PA = partial pressure of gas A XA = mole fraction of gas A Ptotal = total pressure of mixture

Partial 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 Ptotal 0.28 mol XH2= = 0.21 0.28 mol + 0.51 mol + 0.52 mol PH2 = 0.21 x 2.35 = 0.50 atm

In the lab Chemical reaction producing gas eg: NH4NO2 (s)  N2(g) + H2O (l) Determine number of moles (amount) of gas collected?

Partial Pressures When one collects a gas over water, there is water vapor mixed in with the gas. Ptotal = Pgas + PH2O 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)

Gas 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 torr T = 25oC Find: Poxygen Ptotal = PO2 + PH2O Must find Pwater first.

Ptotal = Poxygen + Pwater Partial Pressure Ptotal = Poxygen + Pwater To find Pwater, look in Appendix (p 1111): At 25C, Pwater = 23.76 torr So: 745 torr = PO2 + 23.76 torr PO2 = 745 torr - 23.76 torr = 721 torr