Molecular Weight of Air Jayme Bartz Group Members: Tommy, Kari, and Erica Section 226 Kasun ImaduwageIntroduction: The purpose of this Molecular Weight of Air experiment was to use the ideal gas law and a change in mass equation to find the molecular weight of air. The ideal gas law contains the simple gas laws of Boyle, Charles, and Avagadro to form the most comprehensive gas law in science. This law also shows how the pairs of variables are related. The ideal gas equation provides an overall description of the quantitative physical behavior of ideal gases through the quantities of gas pressure, volume the gas takes up, temperature of the gas, number of moles of gas, and the gas constant R (0.0821 L atm/K mol). This equation was used to find the average number of moles of the substances air and helium, in which case the amount of moles in both substances are equal to each other. PV = nRT The number of moles found was then plugged into a change in mass equation that represents the physical meaning of the change. From this equation the molecular weight of air could be calculated. M = [n*molecular weight of air] – [n*atomic weight of He] Experimental: The procedural information for this experiment was found on the course website. For this experiment, a Mylar balloon was filled with air using a squeeze bulb. Then the balloon’s mass was recorded using a top loading balance. The balloon was blown up and the mass measured two more times. Then the balloon was filled up with helium and its mass was measured. The volume of the balloon was then measured using a bucket of water with a spout on the end. A graduated cylinder was placed under the spout and the balloon pressed down until it was submerged in the water from the bucket. The overflow water’s - collected in the cylinder - volume was then recorded and used as the volume of the balloon. The next property to be collected was the temperature, which was done using the LapPro box and a temperature probe. The probe was stuck into the balloon and held there until there was no change in temperature. The pressure for each trial was given at 1 atm. The helium was then released from the balloon and the process was repeated two more times. Results: The experimental molecular weight of air was found to be 36.75 g/mol. The given theoretical molecular weight of dry air was 28.97 g/mol, giving our calculation a 26.9 percent error. The average change in mass was 1.49 grams. The average number of moles was .0455 mol. The ideal gas equation was manipulated to solve for n. N = PV/RT The change in mass equation was also manipulated to solve for the molecular weight of air. M(air) = change in mass + [n(He) * M(He)] / n(air)Table 1. Masses of Air and Helium Trial 1 Trial 2 Trial 3 Mass Air Balloon 3.23 g 3.24 g 3.25 g Mass Helium Balloon 1.83 g 1.62 g 1.79 g Change in mass 1.40 g 1.62 g 1.46 g Table 2. Ideal gas law components Trial 1 Trial 2 Trial 3 Pressure 1 atm 1 atm 1 atm Volume 1.00 L 1.20 L 1.10 L Temperature 292.6 K 296.3 K 293.8 K # of Moles 0.0416 mol 0.0493 mol 0.0456 mol Table 3. Class Data Molecular Mass of Air Percent Error Team 1 36.8 g/mol 26.9 % Team 2 32.8 g/mol 13.1 % Team 3 36.4 g/mol 25.7 % Team 4 36.0 g/mol 24.3 % Team 5 37.8 g/mol 30.5 % Discussion: The main strategy of the experiment was to combine Parts 1 and 2 because it was necessary to find the pressure, temperature, and volume of each mass before releasing the helium to acquire a new mass recording. Another strategy was to have half of the team obtaining the data and have the other half work on the calculations that the data fit into. It was important, for the experiment, to know that the primary components of air are Nitrogen and Oxygen. Knowing the components yielded the theoretical molecular weight of dry air that the data was compared against. Since 79% of air is Nitrogen and 21% of air is Oxygen, these percentages and the atomic mass of each element can be used to determine that there are 22.13 g/mol of Nitrogen in the air and 6.72 g/mol of Oxygen in air. Added together to be 28.97 g/mol, or the theoretical weight. Knowing that the molar weight of air the results were expected to be under 30 g/mol. It was not expected to have so much percentage error because the obtained results were retrieved with much care. But the goal to find the experimental weight was met but that goal did not coincide with the goal of having a low percent error. The ideal gas assumption also played a role in this experiment. The assumption in the ideal gas law is that gas molecules are all spread out very far from one another and they don’t have any volume. This assumption includes that all interactions between molecules are elastic. Some of the likely sources of error could have stemmed from the recording of the volume and the recording of temperature. When recording volume, the person’s hands who was holding down the balloon could have become slightly submerged and effected the overflow of water. While recording temperature, air from the room could have entered the Helium balloon with the probe and effected the overall internal temperature. If either of these numbers whereincreased it would have increased the answer and overall number of moles which would then be placed in the change in mass equation and increase the experimental weight of air. It is very likely either one or both of these errors occurred in the experiment because the experimental weight was indeed greater than the theoretical weight. Conclusion: The overall goal of this experiment was to use the components of the Ideal gas law in conjunction with the change in mass equation and known theoretical weight of air to find the molecular weight of air. The average molecular weight was found to be 36.75 g/mol. The fundamental approach to the experiment was to apply knowledge about the ideal gas assumptions and use the physical properties of the equation to determine the number of moles in air and Helium. Once that number was deduced, it was added to the change in mass equation, along with the atomic weight of Helium and the average change in mass to then find the molecular weight of air. References: MCAT Questions Website http://mcatquestionoftheday.com/chemistry/truly-ideal-gas-assumptions/ Chemistry 130 Lab Website