How much oxygen is in the air?

              Air is made up of a homogeneous mixture of several different gases, of which oxygen is only a minor constituent. Oxygen is one of the most important elements to animals as well being essential for all combustions and involved in other common natural reactions (rust) The major component (>75%) of air is in fact N₂ which is relatively unreactive, compared to oxygen. Why it is bad to breath pure oxygen

            In this experiment, we are going to determine the % of oxygen in air by exploiting the common reaction of iron rusting. In the presence of oxygen, solid iron, Fe, (we will use steel wool which must be made 'rustable' by soaking in acetone briefly, then vinegar for 2-4 min) will form iron (III) oxide, also known as rust. We will take a sample of air and place some Fe into it and monitor what happens to the amount of air in the system. Any change in volume will correspond to the decrease of O₂ in the system. The reaction will cease when all of the oxygen in the sample has been consumed. By measuring how much of the gas sample reacted and what the total volume of the gas is, we can determine the percent of O₂ in the gas sample.

 Lab report (NOT short form memo) includes nice well formed data tables, including average, standard deviation and % error. Also, sample calculations and a short form memo commentary section. 

    At the beginning of class, the entire section will discuss methods for determining the amount of oxygen in air. The focal point of this lab is to really 'experiment' in trying to achieve good, accurate results. The chemicals are benign and you can be given a lot of freedom to try things out. A bad attitude will make this lab very miserable.

    Your instructor represents the old retired chemist in town who remembers most of his chemistry, but might be sketchy on a few topics. They have certainly never tried this experiment, but have some good ideas. They know where you can find any equipment you might need and can give you suggestions about how much stuff to use, but asking them "Is this a good value?" is futile. On the first day of the lab, before lab partners are picked out, a class discussion will take place to clear up any questions you might have.

Pre-lab questions (on a separate piece of recycled paper) worth 7 points.

1. What is rust? How is it formed? 5th grade answers please.

2. Review the MSDS for iron(III) oxide. Briefly summarize the hazards.

3. Do some research on the what the exact amount of oxygen is in air. This should be reported to at least 4 significant figures. Give the value and COMPLETE reference. 'The internet' is not a valid answer.

4. Come up with an idea to determine the amount of oxygen in air using the steel wool rusting concept. Give a brief procedure as to how you would do it. These ideas will be the starting point for the pre-lab discussion.

Last Edited 27 Oct 08