I think that is a fair method. If you write it up carefully, the teacher should give you at least some points. Now, for maximum points:
1). Measure ... what? For starters, how much are you? See my question? Its meaningless. I didn't ask how old you were, how tall you were, how much you way, how good your grades are, how much money you have, I just asked, How much you? I have to guess you mean how much CO2, and not, how pure it is, or something else. You'll get the most points, when you learn to ask complete, clear questions.
2). How much do you expect there to be? People don't answer things blind, they have to know a little bit about what they're measuring. Google will quickly tell you how much CO2 is in a Pepsi can. Can you weigh that amount easily and accurately?