Rising water post lab Dalton’s law Notes Dalton’s law worksheet Agenda 3/13 Rising water post lab Dalton’s law Notes Dalton’s law worksheet

Dalton’s Law of Partial Pressures

Dalton’s Law of Partial Pressure The total pressure of a mixture of nonreacting gases is equal to the sum of the partial pressures of the individual gases (Ptotal = P1 + P2 + P3 + …)

Ptotal = P1 + P2 + P3 + . . . Ptotal = 2.4 + 6.0 = 8.4 atm PH2 = 2.4 atm PHe = 6.0 atm Ptotal = 2.4 + 6.0 = 8.4 atm

Dalton’s Law Example If we have a flask containing Nitrogen gas, whose partial pressure is 0.78 atm and oxygen gas, whose partial pressure is 0.20 atm, the total pressure in the flask is…. Ptotal = PN2 + PO2 Ptotal = .78 + .20 = .98 atm

Example Problem A mixture of He, Ne and Ar gases have a total pressure of 790 mmHg. If there is 15% Ar, 60% He and 25% Ne, what is the partial pressure of each gas? Ptotal = PHe + PNe + PAr = 790 mmHg 15% Ar = (0.15)(790 ) = 118.5 mmHg + 60% He = (0.60)(790) = 474 mmHg + 25% Ne = (0.25)(790) = 197.5 mmHg P total = 790 mmHg

Step Two: multiply the mole fraction by the total pressure Example What is the partial pressure of carbon dioxide in container that holds 5.0 moles of CO2, 3.0 moles of N2, and 1.0 mole of H2 and has a total pressure of 1.05 atm? Step One: find the mole fraction of CO2 5.0 moles CO2/ 9.0 moles total = 0.56 Step Two: multiply the mole fraction by the total pressure 0.56 x 1.05 atm = 0.58atm

The partial pressure of each gas is equal to the mole fraction (X) of each gas times the total pressure Moles gas x Ptotal = Pgas Total Moles Mole fraction is like a percent

Sample Problem The partial pressure of CO2 in a mixture of gases is 0.8 atm. If the total pressure is 1.05 atm, what is the mole fraction of CO2 in the mixture? PCO2 = 0.8atm Ptotal = 1.05atm (Mole Fraction )Ptotal = PCO2 XCO2 (1.05atm) = 0.8 atm XCO2 = 0.8 atm = 0.76 1.05 atm

Two flasks are connected by a stopcock Two flasks are connected by a stopcock. When the stopcock is closed, flask A contains 3.5 L of nitrogen gas at 2.55 atm and flask B contains 1.5 L of carbon monoxide gas at 0.85 atm. What is the total pressure when the stopcock is open and the gases are allowed to mix? Flask A Flask B 3.5 L N2 1.5 L CO2 2.55 atm 0.85 atm

Step 1: Determine the final volume for the gases. It’s easy Step 1: Determine the final volume for the gases. It’s easy! Add the 2 volumes together Step 2: Use Boyle's law (P1V1 = P2V2) to find the final partial pressure for both N2 and CO2 individually. Step 3: Use Dalton's law of partial pressures to find the total final pressure.

Step 2: Use Boyle’s to find P2 for N2 and CO2 P1V1= P2V2 Step 1: Find V 3.5 +1.5 = 5 L Step 2: Use Boyle’s to find P2 for N2 and CO2 P1V1= P2V2 3.5 L(2.55 atm) = P2 (5L) P2 = 1.785 atm

Step 2 Continued… P2 for CO2 P1V1= P2V2 (1.5L)*(0.85 atm) = (5L) P2 P2 = 0.255 atm Step 3: Dalton’s law PN2 + PCO2 = Ptotal 1.785 atm + 0.255 atm = 2.0 atm