I. Dalton’s Law of Partial Pressures

�The total P exerted by a mixture of gases equals the sum of the P’s of the component gases.

I. Dalton’s Law of Partial Pressures

I. Dalton’s Law of Partial Pressures

�Ptotal = P1 + P2 + P3 …

�Px = (nx RT)/V

�Ptotal = (n1 + n2 + n3 + …)(RT/V)

= ntotal(RT/V)

�The total # of moles (particles) is important, not the species.

I. Dalton’s Law of Partial Pressures

�Ex #1: A 1.00-L mixture of He, Ne, and Ar has a total pressure of 662 mm Hg at 298 K. The partial pressure of He is 341 mm Hg and the partial pressure of neon is 112 mm Hg. What mass of argon is present in the mixture?

I. Dalton’s Law of Partial Pressures

�The mole fraction:

�χa = na/ntotal

�χa = Pa/Ptotal

�Ex #2: A 12.5-L scuba tank is filled with a heliox mixture containing 24.2 g of He and 4.32 g of O2 at 298 K. Calculate the mole fraction, the total pressure and partial pressure of each component.

II. Water Displacement

�A common way to collect a gas is over water.

II. Water Displacement

�Complication: Contamination with water vapor.

II. Water Displacement

�To find actual yield à Dalton’s Law and the Ideal Gas Law.

�Patm = Pgas + PH2O

�PH2O is the vapor pressure of water.

II. Water Displacement

II. Water Displacement

�Ex. #3: Hydrogen gas is collected over water. The barometric pressure today is 755 mm Hg and the temperature is 21°C. What is the partial pressure of hydrogen in atm?

�How many moles of H2 are formed if 100. mL are produced?

PS 6.5

�Read section 5.5

�Ch. 5: #65, 67, 69, 71, 73, 75, 91