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How Gases Work


    1.    Relate density and temperature to molar mass.

Key Terms:


Notes: (13-5)

Everybody has seen balloons at a party floating in the air.  Why do they rise?  What are they filled with?  To answer these questions we must think of the air around us as a fluid and not simply as a gas.  In fluids, objects that are less dense rise and objects that are more dense sink.  This is also true for gases.  Balloons filled at parties are filled with the gas helium (He) because it is less dense than the air that surrounds it.  Below is a list of the most common gases found in the air around us.

Nitrogen (N2) - 78%                    mass - 28amu
Oxygen   (O2) - 21%                    mass - 32amu
Carbon dioxide (CO2) - 1%        mass - 44amu

Looking at the gases that make up the air that surrounds us it is no wonder that a balloon filled with He (4amu) rises.  

The two primary lifting gases are He and H2 - due to their very low densities
Of the two, H2has more lifting power but is not used because of the danger of explosion.

Another way of decreasing the density of a gas is to heat it.  The ideal gas equation tells us that as we heat a gas is occupies more space (expands).  The expansion causes a net drop in density for all of the heated gases in the area.  This is the principle behind a hot air balloon.  

When the air is heated the balloon ascends - decrease in density
As the air cools the balloon descends - increase in density


Diffusion vs Effusion

Diffusion is the property of gases where the particles of gas mix evenly in an enclosed space.  Effusion is the movement of gas through a hole.  

The rate of effusion and diffusion is inversely related to its mass.  Therefore, lighter gases effuse and diffuse faster than heavier gases.
Heat increases the rate of effusion and diffusion.






Last modified: March 07, 2004