Gas Mixtures and Partial Pressures
Transcript of Gas Mixtures and Partial Pressures
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Gases
Gas Mixtures and Partial Pressures
Gas Mixtures and Partial Pressures
How do we deal with gases composed of a mixture of two or more different substances?
John Dalton (1766-1844) - (gave us Dalton's atomic theory)
The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were
present alone
Thepartial pressure of a gas:
The pressure exerted by a particular component of a mixture of gases
Dalton's Law of Partial Pressures:
Pt
is the total pressure of a sample which contains a mixture of gases
P1, P
2, P
3, etc. are the partial pressures of the gases in the mixture
Pt=P
1+ P
2+ P
3+ ...
If each of the gases behaves independently of the others then we can apply the ideal gas law to each gas
component in the sample:
For the first component, n1 = the number of moles of component #1 in the sample
The pressure due to component #1 would be:
For the second component, n2
= the number of moles of component #2 in the sample
The pressure due to component #2 would be:
And so on for all components. Therefore, the total pressure Pt
will be equal to:
Mixtures and Partial Pressures http://www.mikeblaber.org/oldwine/chm1045/notes/Gases/Mixtu
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All components will share the same temperature, T, and volume V, therefore, the total pressure Ptwill
be:
Since the sum of the number of moles of each component gas equals the total number of moles of gas
molecules in the sample:
At constant temperature and volume, the total pressure of a gas sample is determined by the total number
of moles of gas present, whether this represents a single substance, or a mixture
Example
A gaseous mixture made from 10 g of oxygen and 5 g of methane is placed in a 10 L vessel at 25C. What is
the partial pressure of each gas, and what is the total pressure in the vessel?
(10 g O2)(1 mol/32 g) = 0.313 mol O
2
(10 g CH4)(1 mol/16 g) = 0.616 mol CH
4
V=10 L
T=(273+25K)=298K
Pt= P
O2+ P
CH4= 0.702 atm + 1.403 atm = 2.105 atm
Mixtures and Partial Pressures http://www.mikeblaber.org/oldwine/chm1045/notes/Gases/Mixtu
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Partial Pressures and Mole Fractions
The ratio of the partial pressure of one component of a gas to the total pressure is:
thus...
The value (n1/n
t) is termed themole fraction of the component gas
The mole fraction (X) of a component gas is a dimensionless number, which expresses the ratio of the
number of moles of one component to the total number of moles of gas in the sample
The ratio of the partial pressure to the total pressure is equal to the mole fraction of the component gas
The above equation can be rearranged to give:
The partial pressure of a gas is equal to its mole fraction times the total pressure
Example
a) A synthetic atmosphere is created by blending 2 mol percent CO2, 20 mol percent O
2and 78 mol percent
N2. If the total pressure is 750 torr, calculate the partial pressure of the oxygen component.
Mole fraction of oxygen is (20/100) = 0.2
Therefore, partial pressure of oxygen = (0.2)(750 torr) = 150 torr
b) If 25 liters of this atmosphere, at 37C, have to be produced, how many moles of O2
are needed?
PO2
= 150 torr (1 atm/760 torr) = 0.197 atm
V = 25 L
T = (273+37K)=310K
R=0.0821 L atm/mol K
PV = nRT
Mixtures and Partial Pressures http://www.mikeblaber.org/oldwine/chm1045/notes/Gases/Mixtu
11/7/2012
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n = (PV)/(RT) = (0.197 atm * 25 L)/(0.0821 L atm/mol K * 310K)
n = 0.194 mol
1996 Michael Blaber
Mixtures and Partial Pressures http://www.mikeblaber.org/oldwine/chm1045/notes/Gases/Mixtu
11/7/2012