1. Dalton’s Law of Partial Pressures

2. 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

3. 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 + P

4. Dalton’s Law of Partial Pressures (continued)

5. 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

6. 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.

7. Guide to Solving for Partial Pressure

8. 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?

9. Solution STEP 1 Write general equation Ptotal = PO2 + PHe
STEP 2 Substitute data and calculate
Ptotal = mmHg mmHg
= mmHg

10. 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) mmHg
3) mm Hg

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