Lecture11222

Gas State: Density, Molar Mass, Partial Pressure Lecture 11

What about that smoke density?

Why are this smog covering the city?

A less dense gas will lie above a more dense one, in absence of mixing.

The ideal gas law can be easily rearranged to calculate the density of a gas.

Expressing ideal gas density:
PV=nRT
n=m/M (m - mass, M - molar mass)
PV=(m/M) x RT
PM=(m/V) x RT
m/V=d (d-density)
PM=dRT
d = PM/RT

Two important ideas from d=PM/RT:
The density of a gas is directly proportional to its molar mass.
The density of a gas is inversely proportional to its temperature.

A sample problem on calculating gas density.

What allows this balloon fly?

Expressing ideal gas molar mass:
PV=nRT
n=m/M (m - mass, M - molar mass)
PV=(m/M) x RT ; M=mRT/PV
PM=(m/V) x RT
m/V=d (d-density)
PM=dRT
M = dRT/P

Jean Baptiste André Dumas (1800-1884), French scientist

A sample problem on finding the molar mass of a volatile liquid.

Why the ideal gas law holds for any gas or mixture
Gases mix homogeneously (form a solution) in any proportions.
Each gas in a mixture behaves as it were the only gas present (assuming no chemical interactions).

John Dalton (1766-1840), British scientist

Dalton’s law of partial pressures: in a mixture of unreacting gases, the total pressure is the sum of the partial pressures of the individual gases: P total =P 1 +P 2 +P 3 +…

Consider air (N 2 , O 2 , Ar):
P N2 =n N2 RT/V
P O2 =n O2 RT/V
P Ar =n Ar RT/V
P total =P N2 +P O2 +P Ar
P total = n N2 RT/V+ n O2 RT/V+ n Ar RT/V
P total =(n N2 +n O2 +n Ar )RT/V=n total RT/V

Mole fractions and partial pressures in a sample of air: X(  ); X N2 =n N2 /n total X N2 =n N2 /(n N2 + n O2 + n Ar ) P N2 =X N2 P total P O2 =X O2 P total P N2 =X Ar P total

A sample problem on applying Dalton’s law of partial pressures.

The water vapor depends only on the water temperature. When finding the partial pressure of a gas collected above water, we should subtract the water vapor from the total gas pressure.

A sample problem on calculating the amount of gas collected over water.

THE END

Lecture11222

by Uladzimir Slabin on Jul 30, 2008

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