A gas cylinder can hold 1 kg of hydrogen at room temperature and pressure:
If the number of molecules of hydrogen in the cylinder is X, calculate the number of CO₂ molecules in the cylinder under the same conditions of temperature and pressure.
Topic: Mole concept, gas volumes and stoichiometry
Answer
Avogadro's law: equal volumes of all gases, under the same conditions of temperature and pressure, contain an equal number of molecules.
The cylinder is the same cylinder, so the volume is the same, and the question says the conditions of temperature and pressure are the same.
Therefore the number of CO₂ molecules = X.
Number of CO₂ molecules = X
This question looks like a calculation and is not one — it is Avogadro's law stated as a puzzle. Same volume, same temperature, same pressure means the same number of molecules, whatever the gas is.
The trap is to start comparing molar masses: 2 for H₂ against 44 for CO₂, and then scale X by 22. That would be right if the question had said equal MASSES. It says the same cylinder under the same conditions, which fixes the volume, not the mass. The carbon dioxide in that cylinder weighs 22 times as much — but it is the same number of molecules.