# Lecture11222

## by Uladzimir Slabin on Jul 30, 2008

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a supplemental resource for students

a supplemental resource for students

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## Lecture11222Presentation Transcript

• 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