Dalton’s Law of Partial Pressures

Partial Pressure The partial pressure of a gas is the pressure of each gas in a mixture is the pressure that gas would exert if it were by itself in the container

Dalton’s Law of Partial Pressures Dalton’s Law of Partial Pressures indicates that pressure depends on the total number of gas particles, not on the types of particles the total pressure exerted by gases in a mixture is the sum of the partial pressures of those gases PT = P1 + P2 + P3 + .....

Dalton’s Law of Partial Pressures (continued)

Total Pressure For example, at STP, one mole of a pure gas in a volume of 22.4 L will exert the same pressure as one mole of a gas mixture in 22.4 L. V = 22.4 L Gas mixtures 1.0 mole N2 0.4 mole O2 0.6 mole He 1.0 mole 0.5 mole O2 0.3 mole He 0.2 mole Ar 1.0 mole 1.0 atm 1.0 atm 1.0 atm

Scuba Diving When a scuba diver is below the ocean surface, the increased pressure causes more N2(g) to dissolve in the blood. If a diver rises too fast, the dissolved N2 gas can form bubbles in the blood, a dangerous and painful condition called “the bends.” For deep descents, helium, which does not dissolve in the blood, is added to O2.

Guide to Solving for Partial Pressure

Learning Check A scuba tank contains O2 with a pressure of 342 mmHg and He at 855 mmHg. What is the total pressure in mmHg in the tank?

Solution STEP 1 Write general equation Ptotal = PO2 + PHe STEP 2 Substitute data and calculate Ptotal = 342 mmHg + 855 mmHg = 1197 mmHg

Learning Check For a deep descent, a scuba diver uses a mixture of helium and oxygen with a total pressure of 8.00 atm. If the oxygen has a partial pressure of 1280 mmHg, what is the partial pressure of the helium? 1) 520 mmHg 2) 2040 mmHg 3) 4800 mm Hg

Solution Answer: 3) 4800 mmHg STEP 1 Write the equation for the sum of partial pressures. PTotal = PO2 + PHe PTotal = 8.00 atm x 760 mmHg = 6080 mmHg 1 atm STEP 2 Solve for the unknown partial pressure. PHe = PTotal – PO2 STEP 3 Substitute pressures and calculate unknown. PHe = 6080 mmHg – 1280 mmHg = 4800 mmHg