is the mass of pure oxygen gas less than that of the same volume of air at the same temperature/pressure?

Question

I did an experiment in chem class in which i found the mass of a sealed flask with air in it, then replaced the air with oxygen and found the mass of that. The mass of the flask + O2 is less than just the flask. Did i do something wrong?

Answer 1

The typical air composition is: 78% nitrogen, 20.95% oxygen, 0.93% argon, and very small percentage of other gases. At the atmospheric pressure and temperature, air can be very well approximated as the (mixture of) ideal gas/es. Thus one mole of gas always occupies the same volume at the same P and T. The molecular weight of N2 is 28, O2 32, Ar 40. You may calculate the averaged air "molecular" weight, which is less than O2. In another word, O2 density is higher than air, for sure.
When you do experiments, please pay close attention to temperature, composition, wetness, etc.

Answer 2

Compressed air would be made up the same as our atmosphere, which is a little less than 80% nitrogen (N2) and a little more than 19% oxygen (O2) with CO2 making up the biggest part of the ~1% that's left. But it would be under pressure, thus compressed. Pure O2 would be very different than simple compressed air.

Answer 3

The molecules of O,2 are more compact and have a greater
reaction when exposed to a flame or spark. When the air you and I breath is more diluted with mixed particles... If you light a match in a room full of air it will burn till it goes out. Yet light a match in that same room filled with O,2 .... Well I'm sure
you get the idea..... Good luck....( O,2 is smaller) look at the periodic table in your dictionary for a ref....