1. Partial Pressure (Dalton’s Law)
Chapter 11 Gases
11.10
Partial Pressure (Dalton’s Law)
A hyperbaric chamber is used in the treatment of certain diseases.

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 states
that the total pressure
depends on the total number of gas particles, not on the types of particles
exerted by a gas mixture is the sum of the partial pressures of those gases
PT = P1 + P2 + P

4. Illustrating Partial Pressures
The total pressure of two gases is the sum of their partial pressures.

5. 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 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.
Under water, the pressure on a diver is greater than the atmospheric pressure.

6. Solving for Partial Pressure

7. Learning Check
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?

8. Solution STEP 1 Write the equation for the sum of partial pressures.
Ptotal = Po2 + PHe
STEP 2 Solve for the unknown pressure.
Convert the pressure in atm to mmHg to match units.
0.450 atm x 760 mmHg = 342 mmHg = PO atm
STEP 3 Substitute known pressure and calculate the unknown.
Ptotal = 342 mmHg mmHg
= 1197 mmHg

9. Learning Check
For some dives, scuba divers use a mixture of nitrogen and oxygen gases (Nitrox) with a pressure of atm. If the oxygen in a tank of Nitrox has a partial pressure of mmHg, what is the partial pressure of the nitrogen?
1) 520 mmHg
2) mmHg
3) mmHg
A Nitrox mixture is used to fill scuba tanks.

10. Solution STEP 1 Write the equation for the sum of partial pressures.
Ptotal = PO2 + PN2
Ptotal = 8.00 atm x 760 mmHg = mmHg
1 atm
STEP 2 Solve for the unknown pressure.
Ptotal = PO PN2 2
PN2 = Ptotal PO2

11. Solution (continued)
STEP 3 Substitute known pressure and calculate the unknown.
PN2 = Ptotal PO2
PN2 = mmHg mmHg
= mmHg (3)

12. Gases Collected over Water
A gas produced in the
laboratory
usually contains water vapor
PT = Pwater + Pgas
has a partial pressure obtained by subtracting the vapor pressure of water from the PT

13. Gases Collected over Water

14. Learning Check
The decomposition of KClO3 produces O2 gas and solid KCl. If 124 mL of O2 is collected over water at 762 mmHg at 20.0 C, how many moles of O2 are produced? (Partial pressure water at 20.0 C = 18 mmHg.)
2KClO3(s) KCl(s) O2(g)

15. Solution STEP 1 Obtain the vapor pressure of water.
Partial pressure of water at 20.0 C = 18 mmHg
STEP 2 Subtract vapor pressure from total P of gas mixture to give partial pressure of needed gas.
PT = Pwater + Pgas
Pgas = PT - Pwater
PO2 = 762 mmHg - 18 mmHg = 744 mmHg

16. Solution (continued)
STEP 3 Use ideal gas law to convert Pgas to moles or grams of gas collected.
PV = nRT
n = PV = (744 mmHg)(0.124 L) = mol of O2
RT (62.4 L • mmHg)(293 K)
mol • K

17. Gases We Breathe The air we breathe is a gas mixture
contains mostly N2 and O2 and small amounts of other gases

18. Learning Check
A. If the atmospheric pressure today is 745 mmHg, what is the partial pressure (mmHg) of O2 in the air?
1) ) ) 760
B. At an atmospheric pressure of 714, what is the partial pressure (mmHg) N2 in the air?
1) ) )

19. Solution
A. If the atmospheric pressure today is 745 mmHg, what is the partial pressure (mmHg) of O2 in the air?
2) 156
745 mmHg (total) x 21 mmHg O2 = 156 mmHg of O2
100 mmHg
B. At an atmospheric pressure of 714, what is the partial pressure (mmHg) N2 in the air?
1) 557
714 mmHg (total) x mmHg N2 = 557 mmHg of N2

20. Gas Exchange During Breathing

21. Blood Gases
In the lungs, O2 enters the blood, while CO2 from the blood is released.
In the tissues, O2 enters the cells, while CO2 is released into the blood.

22. Blood Gases
In the body,
O2 flows into the tissues because the partial pressure of O2 is higher in the 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 the blood