A Formula Counts Atoms. A Percent Weighs Them.

You'll learnwhy a molecule's atom-count percent and its mass percent give different numbers.

Carbon dioxide is 73% oxygen, and it is also 67% oxygen. Both numbers are right, for the same molecule, on the same day, because the two answer different questions. Twelve short steps; press Next for each one.

12 STEPS · 6 QUICK CHECKS · C = 12, H = 1, O = 16, P = 31 g/mol · v3

one molecule · how much of it · how many of them

Before you start

What you're looking at

Carbon dioxide, CO2, and two empty panels — one for how much of it is oxygen by mass, one by atom count.

The question

How can the same molecule be both 72.7% oxygen and 66.7% oxygen?

Step 1 / 12