AP Chemistry
Breathing Gases
Humans and other animals have evolved to breathe air, a mixture containing 78% nitrogen, 21%
oxygen 1% argon and traces of other gases. As humankind has developed technology, sometimes air is
not suitable for breathing in some places humans venture with their devices. Breathing gases are
artificial mixtures of gases that can support human respiration. They must contain oxygen. A mixture
with lower than 16% oxygen cannot effectively support respiration (at atmospheric pressure). Three
places where breathing gases are used is in flight, medicine and scuba diving.
Mixtures of gases are used in scuba diving. As divers descend below the surface, pressure
increases due to the weight of water above them. This increases the pressure in their lungs. As the
partial pressure of nitrogen increases the amount of nitrogen dissolved in the blood increases. This is
due to Charles' Law. This can cause nitrogen narcosis. http://en.wikipedia.org/wiki/Nitrogen_narcosis
As divers ascend and pressure decreases nitrogen comes out of solution and
http://en.wikipedia.org/wiki/Decompression_sickness
Oxygen toxicity is caused by breathing oxygen at elevated partial pressures. A possible solution to the
problem of too much oxygen or nitrogen in the blood of divers is to reduce or eliminate the amount of
these gases in the mixture used for breathing. Various mixtures of gases are used to overcome these
issues. By adding a third gas to a mixture of nitrogen and oxygen the partial pressures of nitrogen and
oxygen can be lowered to where the above issues are lessened.
Trimix is a mixture of nitrogen, oxygen, and helium. Adding helium is beneficial since helium is
less soluble in water than oxygen and nitrogen. Due to its low mass helium moves out of body tissues
quickly once its partial pressure decreases. The main disadvantage of using helium is its expense. In
addition, if this breathing gas is used to pressurize the diver's dry suit the increased thermal conductivity
of helium decreases the effectiveness of the dry suit.
Heliox is a mix of helium and oxygen. In addition to being used for diving it is used in medicine
since it is easier to breathe than air. Hydreliox is a mixture of oxygen, helium and hydrogen used at very
deep depths since helium can cause high pressure nervous syndrome
http://en.wikipedia.org/wiki/High_pressure_nervous_syndrome. Hydrox is a mixture of hydrogen and
oxygen used at extremely deep depths. Hydrox mixtures have the disadvantage of being flammable or
even explosive.
In addition to diving and medicine, breathing gas is used in aircraft. This is usually pure oxygen.
In military aircraft cylinders of pure oxygen are used. In some aircraft devices that extract oxygen from
the air outside provide oxygen. In commercial airliners oxygen candles containing sodium chlorate use
this reaction: 2NaClO3 = 2NaCl +3 O2 to produce oxygen for passengers to breathe if the aircraft
pressurization is lost. The Lockheed U-2 seen below is an aircraft capable of cruising at 70,000 feet. The
aircraft is capable of climbing very quickly. The pilots of this aircraft breathe pure oxygen in a special
room before flight. They even carry a bottle of oxygen from this room with them on their way to the
aircraft to avoid breathing nitrogen before flight.
Lockheed U-2
Questions:
1. The scuba diver pictured below has a mixture of 60% nitrogen, 20% oxygen and 20% argon in his
tanks. Calculate the partial pressure of each gas when his .500L tanks are pressurized to 5.00 atm
pressure.
2. An aerospace engineer wishes to provide 20 minutes of oxygen to passengers in case of a loss of
cabin pressure. The aircraft concerned carries 400 passengers. Each passenger breathes 1 liter of
oxygen per minute at sea level and 25C. Assume that the 20 minutes will be at sea level. Calculate how
many kilograms of sodium chlorate are needed to provide the oxygen to the passengers of the jet.
3. Why do the U-2 pilots breathe pure oxygen before flight?
4. It is strictly forbidden to lubricate the valves on oxygen tanks with hydrocarbon lubricants. Explain.
5. Use Henry's Law to calculate the solubility of nitrogen in human blood at an altitude of 50,000 feet.
The pressure at this altitude is: Calculate the solubility at sea level as well. Calculate how large a bubble
the nitrogen would from if 1ml of water (pilot blood) was brought from sea level to 50,000 feet and the
nitrogen formed a bubble.
6. A person requires about 1 liter of oxygen per minute. Calculate how many minutes a 50L tank of
trimix with 15% oxygen, 50% nitrogen and 35% helium at 25 atm would last at sea level and 300K.
Calculate how long it would last at 30m under the ocean at 280K.
7. Why do divers use argon to pressurize their dry suits?
(This diver wants a dry suit pressurized by argon.)
8. Calculate by what percent a 100L tank made of 5.00kg of steel is heavier when it is filled with air at 50
atm compared to when it is filled with air at STP.
9. A dive company makes a mistake and fills a 100L tank with an unknown gas. The tank was 5.00kg
when filled with 1atm. of air at300K. It weighs 21.914kg when filled with 50 atm. of unknown gas. Find
the molar mass of the unknown gas. Assume they removed the air before they filled the tank with the
mistake gas.
10. Explain what advantages and disadvantages a mixture of oxygen, nitrogen, helium and hydrogen
would have for diving purposes.