Measuring the Percentage Concentration of Oxygen in Air

Object of the Experiment
We will try to make a measurement of the approximate percentage concentration of oxygen in air.  Air is a mixture of several invisible gases, so we first need to be able to separate just the oxygen from the other gases in an air sample.  As that is being done, there needs to be some kind of a visible change that you can see and measure.

Conducting the Experiment
Basically a moist piece of steel wool is stuck into a glass 100 mL graduated cylinder.  The cylinder is turned upside down and the open end is immersed in a cup of water.  The air in the graduated cylinder is sealed off from the rest of the atmosphere.  The oxygen reacts with the steel wool to form rust and is removed from the air sample (it turns from a gas and becomes part of the rust, a solid).  As oxygen is removed from the sample, water will rise up into the cylinder and its level can be read on the cylinder scale.

If you simply try to immerse the open end of the cylinder in a cup of water you would find that the water doesn't enter the cylinder.  Air pressure keeps the water out. You  want the water to enter partway into the cylinder so that the water level can be read on the cylinder scale.

Note that it isn't that the cylinder is full of air that keeps the water out (as shown above at left), there's actually a lot of empty space in the cylinder.  Rather it is the fact that the air molecules are moving around inside the cylinder at 100s of miles per hour and they strike the water molecules with enough force that the water can't move into the cylinder (the more accurate drawing above at right).  The pressure exerted by the air in the cylinder is what keeps water from rising up into the cylinder.

The solution to this problem is to insert a small piece of flexible tubing into the cylinder as shown above.  If you lower the cylinder into the water while keeping the two ends of the tubing out of the water, water will enter the cylinder.  When the water level can be read on the scale (ideally between the 90 and 100 ml marks), the tubing is removed.  This seals off the air sample and the experiment is underway.

You can carefully rest the cylinder against bottom and side of the cup.  Leave the experiment materials in a location where they won't be disturbed; it may takes several days for all of the oxygen to be completely removed from the air sample.

Periodically lift the cylinder just enough to be able to read the water level.  Don't lift the open end of the cylinder out of the water as this would break the seal and you would need to restart the experiment.  Also make a note of the time.

After some time you will notice that the water level doesn't change between readings.  All of the oxygen in the sample has been removed and the experiment is over.  The figure below shows you one way of removing the steel wool (which should then be discarded).

Straighten a paper clip and then bend about 2/3 rds of it around the end of a pencil to form a corkscrew.  Attach the corkscrew to the end of the pencil and then insert it into the cylinder. With a list twisting the corkscrew will snag the steel wool and you will be able to pull it out of the cylinder and dispose of it.

Data Analysis
Imagine you could measure Ninitial, the total number of air molecules in the air sample at the start of the experiment.  During the course of the experiment oxygen in the air reacts with the steel wool to form rust; oxygen gas is removed from the air sample.  There is a decrease in the number of  air molecules in the sample.  You could determine Noxygen, the number of oxygen molecules, by subtracting Nfinal from Ninitial:

Nfinal is the number of air molecules remaining at the end of the experiment after the oxygen has been removed.

The oxygen concentration could then be found using the following equation:

The problem is that you can't directly measure Ninitial or Nfinal (the air molecules are invisible and there are way too many to be counted anyway).  This experiment is set up in such a way that the pressure of the air in the air sample remains constant even though the oxygen is being removed from the sample (water moves into the graduated cylinder as the O2 is removed which decreases the volume of the air sample).  Because pressure and temperature remain constant the ideal gas law

can be written in the following way:

The air sample volume is directly proportional to N (the term in parentheses remains constant).  A change in N causes the same percentage change in V.  The volume of the air sample is something you can see and measure.  You can determine the oxygen concentration using measurements of volume:

A graph of some sample data and a sample calculation are shown below