Dalton’s Law of Partial
Pressures
What is a partial pressure?
How do we measure partial pressure?
Partial Pressures
• If container contains only one gas, then molecules exert all
the same force.
– Ex. Cylinder of O2 Gas - only O2 molecules
• If container contains several gases, then molecules exert a
part of the total force.
– Ex. Compressed CO2 Gas - both C and O2 molecules
creating forces
In other words….
• The total pressure in a container is the sum of the partial pressures
of all the gases in the container.
Ptotal = P1 + P2 + . . . Pn
• Intermolecular Forces between gas particles are considered to be
negligible.
• Partial Pressure of any gas is independent of the presence of any
other gas in the container.
• Gases in a single container are all at the same temperature and have
the same volume … meaning T and V are constants!
•Partial Pressures measured in the same units as identified from the
total pressure.
Let’s work an example!
Example- A mixture of oxygen, hydrogen and nitrogen
gases exerts a total pressure of 278 kPa. If the partial
pressures of the oxygen and the hydrogen are 112 kPa and
101 kPa respectively, what would be the partial pressure
exerted by the nitrogen.
Ptotal = P1 + P2 + . . . Pn
278 kPa = 112 kPa + 101 kPa + Pnitrogen
Pnitrogen = 278 kPa - (112 kPa + 101 kPa)
Pnitrogen = 65 kPa
Let’s make up our own container of several gases
and determine
the partial pressures of our gases!
Are you a gas
molecule today?
SUMMARY
• Review Concept Map
• Homework- Calculate Partial Pressures
from Worksheet for tomorrow!
Graham’s Law of Effusion
- Discovered by Thomas Graham, 1830s
- relates the rate of effusion to the mass of gas
particles
- rate is inversely proportional to the square
root of the mass of its particles.
- at constant temp and pressure
DEMONSTRATION
Cotton plugs with ammonia and hydrogen chloride are placed
an ends of glass tubing.
What is the white ring being formed?
Why does the white ring form closer to the right end than the
left?
ANSWER! Ammonia (molar mass about 17 g) is diffusing
more quickly than the hydrogen chloride (molar mass about 36
g).
PROBLEMS
1. Compare the rates of effusion of hydrogen and oxygen at
the same temp and pressure.
2. A sample of hydrogen effuses through a porous container
about 9 times faster than an unknown gas. Estimate the
molar mass of the unknown gas.