)the following reaction: 2NO2(g)
N2O4(g) Suppose 15.2 g of nitrogen dioxide (NO2) is placed in a 10.0 L flask at high temperature and then the flask is cooled to 25°C. The total pressure is measured to be 0.500 atm. What partial pressures and mole fractions of each are present?

37
Mother beautiful Partial
Pressure Problem
2NO2(g)
N
2
O
4
(g)
Initially there is
15.2 g/46.01 g mol
-1
= 0.330 mol
of NO
2
and therefore the same number of
moles of nitrogen atoms. If at 25°C there are n
moles of NO
2
and n moles of N
2
O
4
then
because the total number of moles of nitrogen
atoms is a unchanged:
n
NO2
+ 2n
N2O4
= 0.330 mol

Mother beautiful Partial
Pressure Problem

39
Mother beautiful Partial
Pressure Problem
Subtracting n
NO2
+ n
N2O4
= 0.204 mol
From n
NO2
+ 2n
N2O4
= 0.330 mol
n
NO2
= 0.126 mol
n
N2O4
= 0.078 mol
From these results, NO
2
has a mole fraction of 0.62
and a partial pressure of 0.62(0.50 atm) = 0.31 atm;
N
2
O
4
has a mole fraction of 0.38 and a partial
pressure of 0.38(0.50 atm) = 0.19 atm

40
Dalton’s Law of Partial Pressure
•
All the early experiments were done
using air a rather complex mixture of
gases.
•
The ideal gas law holds for this kind of
mixture for two reasons:
•
Gases mix homogeneously (they from
solutions) in any proportions
•
Each gas in the mixture behaves as if it were the only gas present
(
No chemical interactions/reactions occur!
) Volume and
temperature must be held constant.

41
Dalton’s Law
John Dalton
1766-1844

Dalton’s Law of Partial Pressure:
Dalton’s Law of Partial Pressure: The
pressure of a gas mixture is the sum of
the pressures of the individual gases.
(P
total
= P
A
+ P
B) Why does each gas in a mixture contribute to the total pressure in proportion to its mole fraction?

43
Dalton’s Law of Partial Pressure:
Dalton’s Law of Partial Pressure: The pressure of a gas mixture is the
sum of the pressures of the individual gases. SLIDE 52
(P
total
= P
A
+ P
B) Why does each gas in a mixture contribute to the total pressure in proportion to its mole fraction?

44
Dalton’s Law of Partial Pressure: A mixture of gases contains 4.46 mol of neon, 0.74 mol of argon, and 2.15 mol xenon. Calculate the partial pressures of the gases the total pressure is 2.00 atm at a certain temperature. This question could be worded using grams or some other form making you calculate the number of moles.