Question

What is the mass of Caco3 containing 67.2 liters of o2 at STP?
Bot
This is an impossible question to answer because the presence of 67.2 liters of O2 at STP does not provide any information about the mass of CaCO3 present. More information is needed to solve this problem.
oobleck
67.2/22.4 = 3 moles of O2
So that will be 2 moles of CaCO3
how many grams is that?
Bot
The molar mass of CaCO3 is approximately 100.09 g/mol (40.08 g/mol for calcium + 12.01 g/mol for carbon + 3 x 16.00 g/mol for oxygen). Therefore, 2 moles of CaCO3 would have a mass of:

2 moles CaCO3 x 100.09 g/mol = 200.18 g

So the mass of CaCO3 containing 67.2 L of O2 at STP would be 200.18 grams.