Gas Mixtures

Upon studying this section, you should be familiar with the following:


Here, we are referring to a system that has two or more types of gases present.

Dalton's Law: In a gas mixture (example: air, 79%N2, 21%O2), the pressure contribution is directly proportional to its mol fraction (ex. O2 is 21% of air by mole, so its contribution of the air's pressure is also 21%!).

Pi = yi * Ptotal

for our example, PO2 = yO2 * Ptotal

Analogous relation for the volume of a gas (Amagat's Law), Vi = Yi * Ptotal (so O2 would also make up 21% of the total gas volume!)

Example 1

A mixture of 3 gases in a gas cylinder are oxygen, nitrogen, and argon. The partial pressure of nitrogen and argon is 2 atm and .3 atm respectively. If the total pressure of the system is 10 atm, determine the partial pressure and the mole fraction of oxygen.
Note: PTotal = ΣPi, or PTotal = PO2 + PN2 + PAr

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Example 2

Liquid acetone (C3H6O) is fed at a rate of 400 L/min into a heated chamber, where it evaporates into a nitrogen stream. The gas leaving the heater is diluted by another nitrogen stream flowing at a measured rate of 419 m3/min. The combined gases are then compressed to a total pressure P = 6.3 atm gauge at a temperature of 325C. The partial pressure of acetone in this stream is PAr = 501 mmHg. Atmospheric pressure is 763 mmHg. What is the molar composition of the stream leaving the compressor? What is the volumetric flow rate of the nitrogen entering the evaporator if the temperature and pressure of this stream are 27C and 475 mmHg gauge?

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