1. 12.2 DALTON’S LAW OF PARTIAL PRESSURES

2. CONTEXT We have looked at how P, T, V are connected
We know the ideal gas law: PV=nRT
We haven’t really discussed mixtures of gases
If there is a mixture, how much of the total pressure is due to each gas?

3. In a mixture of non-reacting gases
Each gas particle behaves independently
Particles spread themselves out to occupy the space available

4. DALTON’S LAW OF PARTIAL PRESSURES
The total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases (the sum of the pressures that each gas would exert if it were alone).
Ptot = P1 + P2 + P3 + …

7. Partial Pressures of Atmospheric Gases
Air consists of a mixture of gases:
N2 = 78.08% O2 = 20.95%
Ar = 0.93 % CO2 = 0.03%
If Standard atmospheric pressure is kPa, determine the partial pressure of each gas.
P N2 = kPa P O2 = kPa
P Ar = 0.94 kPa P CO2 = 0.03 kPa
PT = PN2 + PO2 + Par + PC02
PT ≅ kPa

8. Gas Collection over Water
Gases can be collected by the downward displacement of water
The gas collected is a mixture of the desired gas and water.
This partial pressure of water vapour will be a component of the total pressure
To find the partial pressure of the desired gas, the vapour pressure of water must be subtracted.
The partial pressure of water vapour depends on the temperature of the water

9. Gas Collection over Water
Hydrogen gas from the reaction of Zinc with hydrochloric acid was collected by the displacement of water. Atmospheric conditions were kPa and 20˚C.
Determine the partial pressure of hydrogen gas.
Ptot = Patm = PH2 + PH2O
PH2 = Patm - PH2O
= kPa kPa
= 98.2 kPa

10. EXAMPLE
Oxygen gas is collected over water at 297 K and an atmospheric pressure of 1.0 atm. The volume of gas is 128 mL. Calculate the mass of O2 obtained. Hint: Water vapour at 297 K is 3 kPa.
T = 297 K
PT = 1.0 atm
= kPa
PH2O = 3 kPa
V = 128 mL = L
m = ?
1. PT = PO2 + PH2O
PO2 = kPa – 3 kPa
PO2 = kPa
2. PV =nRT
n = PV / RT
n = (98.325)(0.128)/(8.314)(297)
n = 5.10 x 10-3 mol
3. m = n x M
m = (5.10 x 10-3)(32.00)
m = 0.16 g

11. Homework
P. 460 #
Worksheet