For our D.E.E.P. project we decided to verify Dalton’s Law. Dalton’s Law is the law of partial pressures. It states that the total pressure of a gas mixture equals the sum of the partial pressures of each gas. The formula for Dalton’s Law is, Total Pressure = P1 + P2 + P3.... +Pn, where P1 is the partial pressure of the first gas in the mixture, P2 is the partial pressure of the second gas, P3 is the Partial Pressure of the third gas, and Pn is the partial pressure of the nth gas.


Our first job was to think of an experiment that improved on experiments done in past years. The error in previous experiments was due mainly to pressure leakage.  After several weeks of drawing and redrawing our project design, we came up with an idea that we thought would work. In our experiment, we decided to burn the oxygen out of two different mixtures of gases to find the pressure of the remaining nitrogen to verify Dalton’s Law. To do this, we used a lighter, a glass jar, a stopper and some plastic tubing.  Since we had made the lighter airtight, we could stick it through the stopper and not have to worry about air leakage as a source of error. We also had a substance called Lime Water, that absorbs carbon dioxide, and since carbon dioxide is made when oxygen is burned, we would not have to worry about this factor in our project. This would improve on past experiments.


In our experiment, we used two gas mixtures: regular air and 60 /40 Nitrox.  Air is made up of about 78% nitrogen and about 21% oxygen. Other gases that make up less than one percent of air, include argon, carbon dioxide, neon, helium, hydrogen and several other trace gases. 60/40 Nitrox (60% nitrogen, 40% oxygen) is a mixture of two gases used in scuba diving when divers want to go deep. A tank of Nitrox was obtained from Scuba Haus.


In our experiment, we used a glass jar which we filled with the gas we were testing. Using a lighter modified to be air tight, (see photos) we burned the oxygen out of the gas mixture. The jar contained 20 ml of lime water to remove the carbon dioxide created when the oxygen was burned.

CO2 + Ca(OH)2 -----> CaCO3 + H2O
gas + aqueous -----> solid + liquid

The pressure of the gas mixture was measured throughout the experiment.
When testing air, our formula, in compliance with Dalton’s Law, was: pressure of air = pressure of nitrogen + pressure of oxygen.  This means that when the oxygen was removed from the air in the jar, the pressure of the air would be expected to decrease by a comparable percentage. The remaining air pressure should be equal to the original partial pressure of the nitrogen.  In three test runs, we recorded an average of 84.8%. This was close to the expected result of 79%. Our trial error was 1.57%. (See results.)  We believe this error came from gas added from the lighter fluid which sometimes caused the stopper to be blown off the jar. Overall, even with the error we encountered, our results appeared reliable, and were consistent with Dalton’s Law.


When testing Nitrox we found an average of 84.7% with an error rate of 1.34%. This result did not conform to Dalton’s Law. We expected the remaining pressure to correspond to the sixty percent nitrogen in Nitrox. In discussing this with our teacher, Mr. Harlan, we learned that the vacuum pump which we used to remove the air from the container before the Nitrox was added, only removed 25 kpa. Kpa is a unit used in vacuums, and measures atmospheric pressure. It stands for Kilopascal. In order to get a full vacuum and pump out all the air, the vacuum has to be able to pump out 100 kpa. However, since ours was only pumping out 25 kpa, most of the air in the jar remained where it was.


Actual Oxygen Content of the Jar
We pumped out 25 kpa, but to get a full vacuum we needed 100 kpa, so .75 regular air was still left. We wanted to calculate the amount of oxygen in the jar after we added the Nitrox. The jar had 25% Nitrox in it, and 75% regular air in it. To find the amount of oxygen in the jar, we multiplied .75 by .21 to find the amount of oxygen from the regular air. We added this number to .25 multiplied by .40, which is the amount of oxygen in the Nitrox. This equaled the percent of oxygen in the whole jar, which was 25%.
.75 (.21) + .25 (.40) = .25


This means that we should have expected a result of 75%, instead of 60%. Our actual average result of 84.7% was higher than expected, probably also due to gas from the lighter. This means that the oxygen content was about 25% rather than 40%. This is also the reason our results for Nitrox were so close to our results for air. In our Nitrox tests, the percent oxygen in the jar was 25%, and in regular air it was 21%, which is only 4% different than the Nitrox. Also, when we closed the valve, and took off the pump, and before we put on the balloon full of Nitrox, there was a space in the tubing between the end and the valve that was filled with regular air. Although this would not have made a big difference, it still may have affected our results.


Our results and Dalton’s Law relate to scuba diving mainly through nitrogen narcosis, also known as being “narced”.  Nitrogen narcosis is an alteration in consciousness almost exactly like getting drunk from drinking too much alcohol. Nitrogen narcosis is caused when a diver is breathing regular air and is diving at a depth of about thirty meters, or one hundred feet or deeper.  As partial pressure increases, the gas, nitrogen, becomes more soluble in blood.  Also, once the gas leaves the scuba tank and enters the body, the pressure on it is equal to the pressure of the water outside of the body.  Because of this, when a diver is about one hundred feet below the surface and the total pressure is about 58.8 pounds per square inch (psi), the nitrogen he or she is breathing, dissolves into the blood stream, where it interferes with the transmission of signals from nerve cell to nerve cell.  This can cause people to have hallucinations and make unintelligent choices that they would not do under normal circumstances. Nitrogen narcosis does not have the same lasting effects as someone who is drunk.  It can easily be reversed by swimming toward the surface.  However, someone suffering from nitrogen narcosis may not be able to make the decision to swim up.  To prevent this from happening, divers should be careful, when diving at depths of one hundred feet or more. Good divers will dive in pairs and will use tests like the “thumbs test” to keep track of their mental status. Divers can also use mixtures of gases such as Nitrox, Trimix and Heliox.  These mixtures of gases have a smaller percentage of nitrogen.


In comparison to prior D.E.E.P. projects on Dalton’s Law, we feel that we have made an improvement in developing and airtight container. We used an airtight valve, instead of a watertight valve. Also, we made an airtight lighter using silicone, which helped our project. Lastly, we used wires wrapped around the tubes to make them airtight.


We have two suggestion for future projects. The pump we used only pumped out 25 kpa. Because of this, only 25% of the gas mixture in the jar was Nitrox, and the other 75% was air. To make this a better experiment, one could pump out 25 kpa and add Nitrox, repeating this several times until almost all of the regular air is gone. The other suggestion we have is to use a lighter that does not release gas when lit.


We learned a lot from doing this project. We gained a better understanding of Dalton’s Law and the importance of making an airtight system.

 

 

 

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