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."

P_{total} = P_{1}
+ P_{2} + . . . P_{n}

P_{t} is the total pressure of a
sample which contains a mixture of gases

P_{1}, P_{2}, P_{3}, 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.

P_{dry gas} = P_{total} -
P_{water 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 (^{o}C)

Vapor Pressure (mmHg)

Temp (^{o}C)

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 ^{o}C.
The pressure of the resultant mixture is 113.0 kPa. What
is the pressure that is exerted by the dry hydrogen alone?

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.

P_{O2}=112
kPa

P_{N2}= ?

P_{H2}=101
kPa

P_{total}=278
kPa

P_{total}
= P_{O2} + P_{N2}
+ P_{H2}

278 kPa = 112 kPa + 101 kPa + P_{nitrogen}

P_{nitrogen} = 278 kPa - (112
kPa + 101 kPa)

P_{nitrogen} = 65 kPa

Questions

Highlight to
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?

P_{total}
= P_{Ne} + P_{Ar}

2.39atm=1.4 atm +P_{Ar}

P_{Ar}
=0.55atm

4. A 450 cm^{3} sample of hydrogen
is collect over water at 12^{o}C. The
pressure of the hydrogen and water vapor mixture is 78.5
kPa. What is the partial pressure of the dry
hydrogen gas?

P_{H2}=78.5kPa(760
mmHg/101.3kPa)=589mmHg

P_{total} = P_{H2}
+ P_{H2O}

P_{total} = 589mmHg
+ 10.52mmHg = 600.mmHg

5. 888 cm^{3} of oxygen are
collected over water with a temperature of 25 ^{o}C.
The total pressure of the gases is 55.8 kPa. What is
the partial pressure of the dry gas?