5.1.2: Oxygen in air

Oxygen content of an air sample

Lab #2 experiment (CHEM 0103)

We need to breathe. When we take up air in our lungs, oxygen travels to all cells in the body and reacts with nutrients in our food such as fats and carbohydrates, providing the energy that keeps us alive. In the air we breathe out, part of the oxygen is replaced by carbon dioxide and water, two of the products of oxidizing nutrients. When we are in regions where oxygen levels are low (e.g. at high altitude), our bodies adopt after a couple of days. In a lecture hall where oxygen levels drop and carbon dioxide levels rise, we get tired faster and start to yawn.

Today, we will measure the oxygen content of an air sample.

You will need equipment from one of your drawers (the technician should provide the equipment and will be responsible for collecting it at the end of lab). The record keeper is responsible for labeling samples so that they don’t get confused with one another.

A chemical reaction that uses up the oxygen in air trapped in a graduated cylinder

In order to measure the oxygen content of your air sample, you will trap a defined volume of air in a cylinder and run a chemical reaction that uses up the air within about 40 minutes. During that time, the air has to stay trapped so you can monitor by how much the volume of your sample decreases, and to prevent fresh air from replenishing the oxygen in you sample.

The chemical reaction is the formation of rust. Oxygen in the gas state reacts with iron to form a solid product (rust), leading to a reduction in volume we can observe (the chemical equation is a simplification, but capture the important result that dioxygen is removed from air):

\[\ce{4Fe(s) + 3O2(g) -> 2Fe2O3(s)}\]

If we run the reaction long enough to remove all of the oxygen, the volume change will indicate how much oxygen was present in the sample at the beginning. The trick is to make the reaction fast enough so that most of the oxygen is used up within our lab session, and to run it in a way that we can observe the volume changes.

We monitor the volume changes by running the reaction in an upside-down graduated cylinder submerged in water. To make the reaction sufficiently fast, we will use a loosely packed sample of steel wool (the high surface area is advantageous in the reaction with a gas) that has been soaked in a 0.2 mol/L acetic acid solution (vinegar is an acetic acid solution with additional molecules that give it flavor. The vinegar acts as a catalyst – similar to salty wet conditions that will make your car rust faster in a New England winter than, say, in the Arizona desert.

Procedure

1. Obtain approximately 0.4 g to 0.5 g of steel wool (record the exact mass), “fluff it up”, soak it in vinegar for a couple seconds and place it in a 25 ml graduated cylinder. Invert the cylinder over the vinegar to remove most of the vinegar (and to make sure the steel wool does not fall out. in the tube.
2. Invert the cylinder and submerge it in water (in a large beaker) with a plastic tube attached to a large syringe reaching into the cylinder. Using the technique demonstrated by the instructor, remove as much air from your trapped sample so that you have 20 mL to 25 mL air in the cylinder and start the timer. Carefully record the volume to a tenth of a mL (lines are 0.5 mL, so you have to estimate), taking a picture sufficiently clear to check the reading later. Work together so that minimal time passes between inverting the cylinder and recording time and volume.
3. Every ten minutes, read off the volume. To do so, hold the cylinder vertically, and lift it until the inside and outside level of water approximately match. During this manipulation, you have to make sure the lip of the cylinder never is above the water level in the beaker, otherwise your gas volume and composition will change, invalidating the experiment.
4. Once you don’t see any change in volume (i.e. less than 0.1 mL difference in 10 minutes), take one more time point. Then, remove the cylinder from the beaker, take tweezers to remove the rusted steel wool and discard it in the provided container.
5. Plot the volume of air over time. Start once you have two data points – this will help to decide when the reaction is almost complete. Using a spreadsheet (e.g. Numbers on the iPad), plot the volume (along the y-axis) against the time (xaxis). This type of graph is called scatter plot or (if you connect the dots) line graph.
6. While still in the lab, calculate the oxygen content: Discuss within your group how you would calculate the content of oxygen in your air sample. As an analogy, imaging you want to know what percentage of M&Ms are red by counting them, eating the red ones and counting them again. Discuss what factors might result in systematic or random errors in your determination (consider the gas laws, the way you measured volumes, the time passing between steps 2) and 3), the volume of the steel wool), and how you might change the experiment to improve it.

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