Dalton’s Law
Ptotal = P1 + P2 + P3 + . . .
Dalton’s Law
The pressure of each component is
called the partial pressure of that
component.
Dalton’s Law
For example: If we have a flask
containing nitrogen gas, whose partial
pressure is .78 atm and oxygen gas,
whose partial pressure is .20 atm, the
total pressure in the flask is
Ptotal = PN2 + PO2
Ptotal = .78 + .20 = .98 atm
Dalton’s Law
Another way to think about this is
through the use of the mole fraction.
The mole fraction is rather like
calculating the percent of each
component.
Dalton’s Law
The mole fraction is written as follows
aN2 = moles N2
total moles
This is basically the percent (by number) of some
element.
Dalton’s Law
We can rearrange Dalton’s Law to use
the mole fraction as follows:
Px = ax (Ptotal)
Or specifically:
PN2 = aN2 (Ptotal)
Dalton’s Law
For example: ordinary air contains
78.084% nitrogen. We might be
interested in knowing what the partial
pressure of nitrogen is in the
atmosphere.
Dalton’s Law
The mole fraction of N2 must be .78084,
so
PN2 = .78084(Ptotal)
If the atmospheric pressure is 640 torr,
PN2 = .78084(640) = 500 torr
Composition of Air
78.084%
20.948%
.934%
.0315%
.001818%
.000524%
.000114%
.0000087%
Nitrogen
Oxygen
Argon
Carbon dioxide
Neon
Helium
Krypton
Xenon