1. 8.8 Partial Pressure (Dalton’s Law)
Our cells continuously use oxygen and produce carbon dioxide. Both gases move in and out of the lungs through the membranes of the alveoli, the tiny air sacs at the ends of the airways in the lungs.
Learning Goal Use Dalton’s law of partial pressures to calculate the total pressure of a mixture of gases.

2. Partial Pressure
The partial pressure of a gas is the pressure that each gas in a mixture would exert if it were by itself in the container. Core Chemistry Skill Calculating Partial Pressure

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. Total Pressure
For example, at STP, 1 mole of a pure gas in a volume of 22.4 L will exert the same pressure as 1 mole of a gas mixture in 22.4 L. Gas mixtures
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 mole N2
1.0 atm
1.0 atm
1.0 atm

5. Total Pressure The air we breathe is a mixture of different gases.
contains mostly N2 and O2, and contains small amounts of other gases.
What we call the atmospheric pressure is actually the sum of the partial pressures of the gases in the air

6. Guide to Solving for Partial Pressure

7. Solving for Partial Pressure
A scuba tank contains O2 with a pressure of atm and He at 855 mmHg. What is the total pressure in mmHg in the tank (volume and temperature are constant)? STEP 1 Write the equation for the sum of the partial pressures. Ptotal = PO2 + PHe STEP 2 Rearrange the equation to solve for the unknown pressure. Convert units to match. ×

8. Solving for Partial Pressure
A scuba tank contains O2 with a pressure of atm and He at 855 mmHg. What is the total pressure in mmHg in the tank (volume and temperature are constant)? STEP 3 Substitute known pressures and calculate the unknown partial pressure. Ptotal = PO2 + PHe Ptotal = 342 mmHg mmHg = 1.20 x 103 mmHg ×

9. Study Check
For a deep dive, a scuba diver uses a mixture of helium and oxygen with a pressure of 8.00 atm. If the oxygen has a partial pressure of 1280 mmHg, what is the partial pressure of the helium (volume and temperature are constant)? A. 520 mmHg B mmHg C mmHg

10. Solution
For a deep dive, a scuba diver uses a mixture of helium and oxygen with a pressure of 8.00 atm. If the oxygen has a partial pressure of 1280 mmHg, what is the partial pressure of the helium (volume and temperature are constant)? STEP 1 Write the equation for the sum of the partial pressures. Ptotal = PO2 + Phe STEP 2 Rearrange the equation to solve for the unknown pressure. Convert units to match.
PHe = Ptotal − PO2 ×

11. Solution For a deep dive, a scuba diver uses a mixture of helium and
oxygen with a pressure of atm. If the oxygen has a
partial pressure of 1280 mmHg, what is the partial pressure of the helium (volume and temperature are constant)?
STEP 3 Substitute known pressures and calculate the unknown partial pressure.
PHe = mmHg – 1280 mmHg
= mmHg or 4.80 × 103 mmHg
The answer is C, 4800 mm Hg.

12. Chemistry Link to Health: Blood Gases
In the lungs, O2 enters the blood, while CO2 from the blood is released.
In the tissues, O2 enters the cells, which releases CO2 into the blood.

13. Chemistry Link to Health: Blood Gases
In the body,
O2 flows into the tissues because the partial pressure of O2 is higher in blood and lower in the tissues.
CO2 flows out of the tissues because the partial pressure of CO2 is higher in the tissues and lower in blood.

14. Chemistry Link to Health: Partial Pressures in Blood
Partial Pressures in Blood and Tissue

15. Gas Exchange During Breathing

16. Concept Map, Gas Laws