Dalton’s Law states that "The total pressure
of a mixture of gases equals the sum of the pressures that
each would exert if it were present alone."
Ptotal = P1
+ P2 + . . . Pn
Pt
is the total pressure of a
sample which contains a mixture of gases
P1, P2, P3
, etc.
are the partial pressures (in the same units) of the gases in the mixture
However, there is an unavoidable problem. The gas saturates
with water vapor and now the total pressure inside the bottle is
the sum of two pressures - the gas itself and the added water
vapor.
WE DO NOT WANT THE WATER VAPOR PRESSURE.
So we get rid of it by subtraction.
Pdry gas = Ptotal -
Pwater vapor
This means we must get the water vapor pressure from
somewhere.
We get it from a table because the water vapor pressure
depends only on the temperature, NOT how big the container is or
the pressure of the other gas.
Temp (oC)
Vapor Pressure (mmHg)
Temp (oC)
Vapor Pressure (mmHg)
-10
2.15
40
55.3
0
4.58
60
149.4
5
6.54
80
355.1
10
9.21
95
634
11
9.84
96
658
12
10.52
97
682
13
11.23
98
707
14
11.99
99
733
15
12.79
100
760
20
17.54
101
788
25
23.76
110
1074.6
30
31.8
120
1489
37
47.07
200
11659
Regents Questions
Example 1 A
sample of hydrogen gas is collected over water at 14.0 oC.
The pressure of the resultant mixture is 113.0 kPa. What
is the pressure that is exerted by the dry hydrogen alone?
Pdry
gas=113.0 kPa(760mmHg/101.3 kPa)=847.8mmHg
Pwater
vapor= From Table=11.99mmHg
Ptotal=?
Pdry gas = Ptotal - Pwater
vapor
Pdry gas = 847.8mmHg
- 11.99mmHg
Pdry gas =835.8mmHg
Example 2 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.
PO2=112
kPa
PN2= ?
PH2=101
kPa
Ptotal=278
kPa
Ptotal
= PO2 + PN2
+ PH2
278 kPa = 112 kPa + 101 kPa + Pnitrogen
Pnitrogen = 278 kPa - (112
kPa + 101 kPa)
Pnitrogen = 65 kPa
Questions
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Reveal Answers
3. A mixture of neon and argon gases exerts
a total pressure of 2.39 atm. The partial
pressure of the neon alone is 1.84 atm, what is the
partial pressure of the argon?
Ptotal
= PNe + PAr
2.39atm=1.4 atm +PAr
PAr
=0.55atm
4. A 450 cm3 sample of hydrogen
is collect over water at 12oC. The
pressure of the hydrogen and water vapor mixture is 78.5
kPa. What is the partial pressure of the dry
hydrogen gas?
PH2=78.5kPa(760
mmHg/101.3kPa)=589mmHg
Ptotal = PH2
+ PH2O
Ptotal = 589mmHg
+ 10.52mmHg = 600.mmHg
5. 888 cm3 of oxygen are
collected over water with a temperature of 25 oC.
The total pressure of the gases is 55.8 kPa. What is
the partial pressure of the dry gas?
