Another sample of gas at STP with a volume of 3.0 liters will contain the same number of molecules as 3.0 liters of H2, due to Avogadro's law and the concept of molar volume at STP.
The question is asking about the relationship between volumes of gases and their corresponding number of molecules at standard temperature and pressure (STP). To answer this, we use Avogadro's law, which states that equal volumes of gases at the same temperature and pressure contain an equal number of molecules.
At STP, one mole of any gas occupies 22.4 liters. So, if 3.0 liters of H2 contains a certain number of molecules, another gas will contain the same number of molecules if it also occupies 3.0 liters at STP. This is due to the fact that the molar volume of a gas at STP is 22.4 L/mol.
Therefore, another sample of gas that is also at STP and has a volume of 3.0 liters will contain the same number of molecules as 3.0 liters of H2.