This paper presents a simplified determination of the percent oxygen in air with simple and special designed apparatus. The method is based on a consumption of oxygen resulting in reduced pressure and causing water to be drawn in to the inverted flask. This experiment should be very safe, rapid, and simple. The experiment, the combustion of ethyl alcohol with oxygen in air, can either be demonstrated by chemistry teachers or be performed easily and exactly (overall average 20.77 to 20.81 % O2) by students in the student laboratory. In this way, a fountain will be produced, utilizing the siphoning effect of reducing the pressure in the inverted flask. The maximum volume of water, which is drawn into the inverted flask from the Erlenmeyer flask below by creating an interesting fountain, is reached in 5-10 minutes.
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