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  Determination of the Percent Oxygen in Air, Chemistry tutorial

Introduction:

The air is a homogeneous mixture of gases like nitrogen, oxygen, argon and trace amounts of other elemental gases and carbon-dioxide. The amounts of each and every gas can be evaluated both by weight and volume to find out the percent composition. In this experiment, the students will assess gas volumes by employing gas measuring burettes. As gases are extremely sensitive to changes in temperature and pressure, the students must cautiously note the atmospheric pressure, laboratory and water temperatures.

Theory:

Consider for a moment the air which you breathe. As the time of the ancient Greek philosophers, people have realized that the air is critical to life, although by little understanding of why. We now recognize that the most common gases in air are nitrogen (78%), oxygen (around 21%), and argon (nearly 1%). Other molecules are present in the atmosphere too, however in extremely small quantities.

In this laboratory experiment, we will perform a method to confirm the oxygen content of:

4Fe (s) + 3O2 (g) → 2Fe2O3 (s)

Anybody who has witnessed rust on a car, bicycle or barbed wire fence recognizes that this reaction takes place spontaneously, although the rate can be extremely slow. To hasten the process and complete the data collection in one laboratory period, we will first 'activate' the iron by washing it by acetic acid. It is assumed that a small quantity of acid catalyzes the reaction, although the procedure is not well understood. On the other hand, surplus of acid could interfere with the results via reacting with the iron itself, to form the hydrogen gas. 

As the oxygen in air reacts by iron to form solid iron(III) oxide, the volume of trapped air must reduce and water will enter the test tube. This change in volume is equivalent to the volume of oxygen used in the reaction.

Supposing that the length of the test tube is proportional to its volume and that the change in length of the column of air in the test tube is due only to the elimination of oxygen, the percentage of oxygen can be found out by computing the change in the volume of air in the test tube.

% O2 = Constant water level reading (mm)/tube length (mm)

To make sure that all the oxygen is fully reacted, iron will be present in surplus. The second question which you will attempt to answer in this experiment is what is the optimal quantity of iron to be employed? To determine the answer to this question, it will be essential to perform a sequence of experiments. Instead doing these all yourself, you will pool data by others in the class.

Procedure:

1) Fill a 15 cm test-tube fully with water. Pour the water into a 100 ml beaker and weigh. Record the temperature of the water.

2) Assess the length in millimeters of the test-tube. Measure to the point halfway between the end and starting of the rounded end. Attach a plastic metric ruler by two rubber bands in such a way that the metric length starts at the lip of the tube. The rubber bands must be positioned around the bottom half of the test-tube, leaving your view of the top half unobstructed.

3) Fill a 400 ml beaker around 3/4 full with water.

4) Get a small piece of steel wool from the front bench. Evaluate and record the mass. Steps 5 to 7 must be done fast while working in the fume hood. We will require the weighed piece of steel wool, forceps and the test-tube used in step 1.

5) Holding the steel wool by forceps, rinse completely with acetone. (This will eliminate any oils from the surface of the steel wool.) Shake off surplus acetone in the acetone waste bucket.

6) Soak the steel wool in a 50:50 vinegar or water mixture for around 1 minute, ensuring that all of the steel wool is under the surface of the solution. Take away the steel wool and shake off surplus solution in the acetic acid waste bucket.

7) Pull apart the steel wool to raise the surface area and insert it to the bottom of the test-tube. Push the steel wool loosely into the test-tube by a glass stirring rod.

8) Working back at your station, cover the end of the test-tube by your finger and quickly invert the test-tube assembly into the beaker of water, removing your finger once the opening of the test-tube is beneath water. If essential, adjust the 0.0 mm ruler mark to the water level within the tube. Record the time.

9) After five minutes, move the test-tube in such a way that the water level within the test tube is equivalent to the water level within the beaker. We will find this simplest to accomplish by holding the ruler against the side of the beaker. Evaluate and record the height of the water in the test-tube and then rest it on the bottom again.

10) Measure and record the height of water in the test-tube every 5 minutes by employing the method in step 9 till the water level stops changing. Take two or three readings at the last constant level.

11) Take away the wire from the test-tube, record its color, reject it and clean the test-tube.

12) Repeat the steps 3 to 11 by a fresh piece of steel wool.

Computations:

Steps 1 to 3 might be done while collecting data (above) and should be completed before leaving the lab.

1) For each and every trial, make an Excel table and record the water level (mm), time (minutes) and percent change in the water level reading.

2) Graph the percent change in the water level reading versus time. (That is, curves from both trials might be recorded on the similar graph.

3) Record your value for the mass of iron used the percent volume of oxygen in air and the time it took to arrive at constant volume on the class summary table in lab. Before leaving, copy the class information to your lab note-book.

4) Compute the class average and standard deviation for the percent volume of oxygen.

5) Compute the total volume of the test-tube, based on the mass of water measured in step 1 and the density. The density of steel is approximately 7.9 g/mL at 20°C. What percentage of the tube volume was occupied via the steel wool for:

a) The lightest piece of steel wool?

b) 1.0 g of steel wool?

c) Heaviest piece of the steel wool?

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