Mole percent oxygen in air lab write up: Experiment 10 PDF

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Lab write up for experiment 10....

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Mole Percent Oxygen in Air Experiment 10

Abstract In this experiment, the mole percent of oxygen in air was found experimentally by confining a solution that reacts with oxygen and air in a contained volume, and measuring the decrease in gas volume that occurred. Using the Ideal Gas Law, the decreased volume of water saturated oxygen was converted to moles decreased, and the moles of water vapor subtracted from this amount to produce the dry oxygen absorbed. This amount out of the total air volume decreased produced the mole percent of oxygen in air, which was 22.7%. The theoretical value of oxygen in air is about 21%, so there was a percent error of 8.1%, likely due to escaped gas. Introduction When the molecule [Co(NH3)5H2O]2+ is exposed to air, it instantly reacts with the oxygen to form [(NH3)5-Co-O2-Co-(NH3)5]4+. This reaction can be reversed by changing the conditions surrounding the experiment, and is often because of its relation to oxygen binding to hemoglobin, which is reversible depending on conditions as well. This experiment measures the amount of oxygen in air in moles by confining [Co(NH3)5H2O]2+ and air in a given volume, and measuring the decrease in volume as a result of the oxygen in the air reacting with the complex. The air is saturated with water vapor, but it is possible to find the moles of dry oxygen by using the partial pressure of water vapor at a measured temperature to find out how many moles of water vapor were included in the initial measurement, and then subtracting this amount from that measurement. The percent of oxygen in air can be found by dividing it by the total amount of air that was contained in the volume before the oxygen was removed. It is common knowledge that the theoretical percentage of Oxygen in air is about 21%. Procedure First a gas burette and leveling bulb were supported on two ring stands and connected together with rubber tubing. The gas burette was filled with water and raised until the water drained into the leveling bulb, reaching the lowest point visible in the burette. A screw clamp was closed tightly on the tubing, and the burette was lowered. The leveling bulb was filled with water and then, using a funnel, 5 mL 1 M CoCl2 and 5 mL 6 M NH3 were poured into the burette. The burette was immediately stoppered. After waiting for the solution on the walls to drain, the initial position of the solution on the burette was recorded. The solution reacted with the oxygen for 10 minutes, and every 2 minutes the gas burette was removed from the ring stand and tipped back and forth horizontally to allow maximal contact of the solution with oxygen. The screw clamp was then loosened and then removed slowly, allowing the water from the bulb to move into the gas burette. The level of the water in the leveling bulb was adjusted to match the level of the solution in the burette by moving it up and down the ring stand. The upper level of the solution in the gas burette was recorded. The screw clamp was placed back on the tubing, and a rubber band was placed around the top of the burette to mark the bottom of the rubber stopper, which was then removed. The temperature of the solution was recorded.

Next, 60 mL of water was obtained in a 100 mL beaker and the mass was measured and recorded. To measure the volume of the “empty” space above the solution in the burette, water from the beaker was added until it reached the rubber band. The mass of the beaker and the remaining water was measured and recorded and the volume of the water added was determined assuming a density of 1.00 g/mL for the water.

Results and Observations Results: Level of solution before reaction (mL) Level of solution after reaction with added water (mL) Temperature of solution (°C) Volume of oxygen and water vapor removed (mL) Mass of beaker and water (g) Mass of beaker and leftover water (g) Total volume air* (mL)

9.6 21.82 22.7 12.22 130.097 91.149 38.948

*Total volume = (130.097 – 91.149) g H2O x 1 mL/1 g = 38.948 mL air

Observations: CoCl2 and NH3 reaction

Addition of water to solution in burette

The NH3 solution was clear and the CoCl2 solution was red. Upon mixing the solutions in the burette, the solution turned blue and was very cloudy. After mixing the solution in the burette by tipping it horizontally, the solution became swamp green and frothy. Upon adding the water to the solution inside the burette, many bubbles formed and a dark teal solid floated to the top of the solution. The solution appeared cloudy green at the bottom and transparent in the middle.

Calculations: Moles of Oxygen and Water Vapor: n= PV/RT T = 22.7 °C  295.85 K (736 mmHG)(.01222 L) n= =4.87e10 -4 moles oxygen and water vapor (62.3637 LmmHg /Kmol )(295.85 K) Moles of Water Vapor P at 22.7°C is approximately 21.07 mm Hg according to Table 10.1 in the lab instructions. (21.07 mmHG)(.01222 L) =1.40e10 -5 moles water vapor n= (62.3637 LmmHg /Kmol )(295.85 K) Moles of Dry Oxygen

4.87e10 -4 moles - 1.40e10 -5 moles = 3.47e10-4 moles O2 Moles of Total Air (736 mmHG)(.03895 L) n= =1.56e10 -3 moles total air (62.3637 LmmHg / Kmol )(295.85 K) Mole Percent Oxygen in Air 1.56e10 -3 moles total air = .223 x 100 = 22.7% 3.47e10-4 moles O2 ÷ Results of Calculations Oxygen and Water Vapor (moles) Water Vapor (moles) Oxygen (moles) Total Air (moles)

4.87e10 1.40e10 3.47e10-4 1.56e10

-4 -5

-3

Percent Error: Theoretical yield – actual yield/theoretical yield 21% - 22.7%/21% = 8.1 % Discussion & Error Analysis The results of this experiment found that the mole percent of Oxygen in air is 22.7%. The theoretical value for the percent of Oxygen in air is about 21%, so there was a percent error of 8.1%. This is slightly high, and was likely caused by errors that occurred during the experiment. One source of error for this experiment was that when the CoCl2 and the NH3 were mixed, the solution was exposed to the open air for a few seconds before the stopper could be placed on the burette, making the measurements a little bit inaccurate. Also, it is possible that the clamp could have let some air escape, which would affect the pressure and volume of air inside the burette as well. Another source of inaccuracy could be that when leveling the bulb and burette so that the water level was about the same, it was hard to tell if they were at exactly the same level. Another major source of error could be that when adding the water to the burette to measure the total volume of air that was inside, it was very hard to make sure the water added was exactly to where the rubber band was placed. This was because there was foam at the top of the solution that made it hard to see where it actually leveled off....