1. Ch – Gases
III. One More Law
Dalton’s Law
Pages 408 – 410

2. Ptotal = P1 + P2 + ...Pn A. Dalton’s Law (of partial pressures)
The total pressure of a mixture of gases equals the sum of the partial pressures of the individual gases
Ptotal = P1 + P Pn

3. A. Dalton’s Law
A sample of air has a total pressure of atm. The mixture contains only CO2, N2, and O2. If PCO2 = 0.12 atm and PN2 = 0.70 atm, what is the partial pressure of O2?
GIVEN:
PO2 = ?
Ptotal = 1.00 atm
PCO2 = 0.12 atm
PN2 = 0.70 atm
WORK:
Ptotal = PCO2 + PN2 + PO2
1.00 atm = 0.12 atm atm + PO2
PO2 = 0.18 atm

4. What you are trying to find
A. Dalton’s Law
(over water)
When H2 gas is collected by water displacement, the gas in the collection bottle is actually a mixture of H2 and water vapor
Water exerts its own pressure known as water-vapor pressure
So, to determine total pressure of gas and water vapor inside a container, make total pressure inside bottle = atmosphere and use this formula:
What you are trying to find
Look up on table
Always known
Ptotal = Pgas + PH2O

5. A. Dalton’s Law
Hydrogen gas is collected over water at 22.5°C. Find the pressure of the dry gas if the atmospheric pressure is 94.4 kPa.
The total pressure in the collection bottle is equal to atmospheric pressure and is a mixture of H2 and water vapor.
GIVEN:
PH2 = ?
Ptotal = 94.4 kPa
PH2O = 20.4 mm Hg
= 2.72 kPa
WORK:
Ptotal = PH2 + PH2O
94.4 kPa = PH kPa
PH2 = 91.7 kPa
Look up water-vapor pressure for 22.5°C, convert to kPa
Sig Figs: Round to lowest number of decimal places.

6. A. Dalton’s Law GIVEN: Pgas = ? Ptotal = 742.0 torr PH2O = 42.2 torr
A gas is collected over water at a temp of 35.0°C when the barometric pressure is torr. What is the partial pressure of the dry gas?
The total pressure in the collection bottle is equal to barometric pressure and is a mixture of the “gas” and water vapor.
GIVEN:
Pgas = ?
Ptotal = torr
PH2O = 42.2 torr
WORK:
Ptotal = Pgas + PH2O
742.0 torr = Pgas torr
Pgas = torr
Look up water-vapor pressure for 35.0°C.
Sig Figs: Round to least number of decimal places.