How Much Oxygen is There in Air?

This is an experiment to measure how much oxygen is in the air. It's an easy one-evening experiment. Get some of those hand-warmer pouches. They come sealed in plastic film, and inside are two cloth bags, one for each hand. They cost about $1 at hardware stores. (Trouble finding them?) The pouches have charcoal and iron powder in them. The iron reacts with oxygen in the air to make rust. This uses up all the oxygen. It also makes heat to keep your hands warm. These things are oxygen-eaters! They come sealed in plastic so that after they use up the little bit of oxygen that's in the package with them, they stop reacting until you open the package. Here's what a company that makes them says about how they work. Tape one of the pouches inside the bottom of a cylindrical drinking glass. Duct tape works well (doesn't it always?). Then put the glass upside down in a shallow plate of water, like this. If you don't have duct tape, you can use a pencil or something to keep the pouch from falling into the water, like this. As the hand-warmer uses up the oxygen in the air inside the glass, the water level rises up into the glass, like this. When all the oxygen is gone, the water stops rising. You can see how much the water level rose, as a fraction of the total amount of air originally in the glass. That's the fraction of the air that was oxygen. It takes a few hours to finish using up the oxygen, but you can see the water rising after about 20 minutes. What percentage of our air is oxygen? Some people have written me that they have tried more advanced experiments, like comparing indoor air to outdoor air. If you do this, keep in mind that cold air will expand a bit as it warms up, and this can bias your results. So you need to start with both samples of air at room-tempertature air. Some people have also written me that the "cylidrical"glass in my photos isn't very cylidrical! True; use the most straight-sided glass you can find. Or, can you think of some way to get an accurate result for what fraction of the volume of air in the glass got used up, even if the glass is any odd shape? Here's the most popular question: How do I tell from the water level how much of the air was oxygen? Well, for instance, if the water rose 1/3 of the way up the glass, then 1/3 of the air in the glass was oxygen. Would you like to compare your answer to the "accepted" answer? Here is a chart of the composition of the atmosphere. --------------------------------------------------------------------------------