1. Rising water post lab Dalton’s law Notes Dalton’s law worksheet
Agenda 3/13
Rising water post lab
Dalton’s law Notes
Dalton’s law worksheet

2. Dalton’s Law of Partial Pressures

3. Dalton’s Law of Partial Pressure
The total pressure of a mixture of nonreacting gases is equal to the sum of the partial pressures of the individual gases
(Ptotal = P1 + P2 + P3 + …)

4. Ptotal = P1 + P2 + P3 + . . . Ptotal = 2.4 + 6.0 = 8.4 atm
PH2 = 2.4 atm
PHe = 6.0 atm
Ptotal = =
8.4 atm

5. Dalton’s Law Example
If we have a flask containing Nitrogen gas, whose partial pressure is 0.78 atm and oxygen gas, whose partial pressure is 0.20 atm, the total pressure in the flask is….
Ptotal = PN2 + PO2
Ptotal = = .98 atm

6. Example Problem
A mixture of He, Ne and Ar gases have a total pressure of 790 mmHg. If there is 15% Ar, 60% He and 25% Ne, what is the partial pressure of each gas?
Ptotal = PHe + PNe + PAr = 790 mmHg
15% Ar = (0.15)(790 ) = mmHg +
60% He = (0.60)(790) = mmHg +
25% Ne = (0.25)(790) = mmHg
P total = mmHg

7. Step Two: multiply the mole fraction by the total pressure
Example
What is the partial pressure of carbon dioxide in container that holds 5.0 moles of CO2, 3.0 moles of N2, and 1.0 mole of H2 and has a total pressure of 1.05 atm?
Step One: find the mole fraction of CO2
5.0 moles CO2/ 9.0 moles total = 0.56
Step Two: multiply the mole fraction by the total pressure
0.56 x 1.05 atm = 0.58atm

8. The partial pressure of each gas is equal to the mole fraction (X) of each gas times the total pressure
Moles gas x Ptotal = Pgas
Total Moles
Mole fraction is like a percent

9. Sample Problem
The partial pressure of CO2 in a mixture of gases is 0.8 atm. If the total pressure is 1.05 atm, what is the mole fraction of CO2 in the mixture?
PCO2 = 0.8atm Ptotal = 1.05atm
(Mole Fraction )Ptotal = PCO2
XCO2 (1.05atm) = 0.8 atm
XCO2 = atm = 0.76
1.05 atm

10. Two flasks are connected by a stopcock
Two flasks are connected by a stopcock. When the stopcock is closed, flask A contains 3.5 L of nitrogen gas at 2.55 atm and flask B contains 1.5 L of carbon monoxide gas at 0.85 atm. What is the total pressure when the stopcock is open and the gases are allowed to mix?
Flask A Flask B
3.5 L N L CO2
2.55 atm atm

11. Step 1: Determine the final volume for the gases. It’s easy
Step 1: Determine the final volume for the gases. It’s easy! Add the 2 volumes together
Step 2: Use Boyle's law (P1V1 = P2V2) to find the final partial pressure for both N2 and CO2 individually.
Step 3: Use Dalton's law of partial pressures to find the total final pressure.

12. Step 2: Use Boyle’s to find P2 for N2 and CO2 P1V1= P2V2
Step 1: Find V
= 5 L
Step 2: Use Boyle’s to find P2 for N2 and CO2
P1V1= P2V2
3.5 L(2.55 atm) = P2 (5L)
P2 = atm

13. Step 2 Continued… P2 for CO2
P1V1= P2V2
(1.5L)*(0.85 atm) = (5L) P2
P2 = atm
Step 3: Dalton’s law
PN2 + PCO2 = Ptotal
1.785 atm atm = 2.0 atm