Chemistry and Gases Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 29, 2011


Although it may seem like it, the air we breathe is not limitless. It reaches out about 30 kilometers from the surface of the Earth, but is only breathable to humans to about 14,000 feet or 4.3 kilometers. The air we breathe, in fact, is not just one gas, but several.

Outside and inside air is made up of roughly 78% nitrogen, 21% oxygen, and 1% argon with a smattering of 3–4% water vapor, carbon dioxide, sulfur dioxide, and the list goes on depending on where in the world you live.

Carbon Dioxide

Something important to consider is that the amount of polluting (chemicals not found in high levels in nature) gases in the atmosphere is rising. Levels at the times of the pyramids with fires and local industry, were around 80–100 parts per million (ppm), in 1900, the levels of CO 2 were less than 300 ppm. Today carbon dioxide levels approach 400 ppm. Since the human body does not do well breathing low levels of oxygen or pollutants, this is a significant problem. Not only is the air no longer pure in many parts of the world, but increasing levels of carbon dioxide add to the problem of rising global temperatures.

When gases expand and mix with other gases to fill available space, it is called diffusion . This is how environmentalists measure the levels of industrial gases in the air. When there is a gas release, they measure the amount of gas in parts per million or parts per billion. From these measurements, they can figure out the released amount and whether or not the diffused concentration is harmful to humans.

Kinetic Gas Theory

Gas molecules are always on the move. They are always bouncing off each other and other things like the walls of a container, or people, places, and things. They are super charged with energy. When scientists talk about this crazy motion of gases, they call it kinetic energy .

The kinetic energy of gases can be calculated. It is equal to one-half the mass (m) of the sample multiplied by the velocity squared ( v 2 ):

Kinetic energy = ½ m v 2

Then if a scientist has a sample of a known mass in a container with the molecules bouncing all over the place, the kinetic energy can be calculated using the equation above.

Chemists calculate the kinetic energy of gases based on their temperature, which in turn affects their velocities. The average kinetic energy of a gas molecule depends on the absolute temperature of the gas.

Gas Pressure

The atmosphere contains different gases as we learned earlier. These gas molecules collide with everything in our world, all the time. The Earth’s gravity affects the force with which gas molecules hit people, objects, and each other. Gravity’s pull on gas molecules decreases when molecules get farther and farther away from the Earth. Their weight is decreased without the constant tug of gravity although their mass remains the same. It is the same reason why astronauts are weightless in space and weigh only about a third of their weight on the moon (with a lot less gravity) as they would on Earth.

Atmospheric pressure is caused by the weight of the air per unit of area.

Since the first experiments to find this difference in pressure were performed with mercury and tall glass tubes by Italian scientist Evangelista Torricelli, the standard unit for pressure was called the torr . Experiments found that at sea level, the lowest land location to measure, that atmospheric pressure is equal to 1 atmosphere (1 atm).

1 atm = 760 mm Hg = 760 torr

Note: When engineers and mechanics calculate pressure, they sometimes use the units pounds per square inch (psi).

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