Please pick up

a copy of two problem sets

• Wet Gases problem set• Gas Bulb Problems problem set

Gaseous Mixtures

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

Gaseous Mixtures

Reading Assignment:• Zumdahl, Chapter 5.5, 17.4

Gas Mixtures• Mole fraction• Dalton's Law of Partial Pressures• Vapor pressure • Wet gases

Atmospheric Composition

Dry air

78% N2, 21% O2, 1%Ar Normal air

N2, O2, Ar, 0.03% CO2, 1.6% H2O

Mole Fraction

The ratio of moles of a compound in a mixture to the total moles in the mixture.

A= + + +mol

mol mol molA

A B C ...

A gas sample contains 3 moles of hydrogen and 2 moles of oxygen.

What is the mole fraction of each gas?

Mole Fraction

H2

=moles H2

moles H2 + moles O2

=3

3 + 2= 0.60

O2

=moles O2

moles H2 + moles O2

=2

3 + 2= 0.40

The sum of the mole fractions in a mixture

equals exactly 1

Partial Pressure

The pressure a fixed amount of gas would exert if it were alone in a given volume at a given temperature.

Gas Bulb3 liters, 2 atm 2 liters, 1 atm

What will happen when the stopcock is opened?

Gas Bulb3 liters, 2 atm 2 liters, 1 atm

What will be the pressure of the orange atoms?

Hint: Use Boyle’s Law

Gas Bulb3 liters, 2 atm 2 liters, 1 atm

P · V = constant

2 atm · 3 L = 6 L· atm

Porange · 5 L = 6 L· atm

Porange = 1.2 atm

Partial pressure

Dalton's Law of Partial Pressures

The total pressure of a non-reacting gaseous mixture is the sum of the partial pressures of each gas in the mixture.

Ptotal = PA + PB + PC + ...

Gas Bulb3 liters, 2 atm 2 liters, 1 atm

What is the total pressure in the left bulb?

1.6 atm

What is the total pressure in the right bulb?

1.6 atm

Dalton's Law of Partial Pressures

What is the mole fraction of each gas?

PA = Ptotal · A

What is the partial pressure of each gas?

Vapor Pressure

The pressure exerted by the vapor of a substance when in equilibrium with the substance.• Even below their boiling point, liquids

evaporate or vaporize and solids sublime.

Vapor Pressure

• If a container is closed, then only a certain amount of liquid will evaporate until the air is saturated with the vapor.

Adding additional liquid does not increasethe amount of vapor in the air.

Vapor Pressure

• Boiling occurs when the vapor pressure of a liquid equals or exceeds the restraining pressure (e.g., atmospheric pressure)

Is it possible forwater to boil at room

temperature?

Vapor Pressure of Water

0

20

40

60

80

100

0 10 20 30 40 50

Temperature (°C)

Pre

ssu

re (

torr

)

Increases non-linearly with increasing temperature.

Wet GasesGases trapped byliquid displacement.

P P Ptotal gas water vapor= +

Is the pressure of the trapped gas equal to,less than or greater than atmospheric pressure?

Common GasesRoom Temperature, One Atmosphere Pressure

Monatomic: He, Ne, Ar, Kr, Xe, Rn Polyatomic

• Elements: H2, N2, O2, O3, F2, Cl2• Compounds: CO, CO2, NO, N2O, NO2,

N2O4, N2O5, CH4, C2H6, C2F4, etc.