Final answer:
At standard temperature and pressure, all gases occupy the same volume for an equal number of moles due to Avogadro's law. Therefore, both hydrogen (H2) and oxygen (O2) will have the same volume if the number of moles is kept constant, despite differences in molecular weights.
Step-by-step explanation:
When comparing different gases at standard temperature and pressure (STP), which for many cases is defined as 0°C (273.15 K) and 1 atm (101.325 kPa), all gases will occupy the same volume provided the number of moles is constant. Specifically, according to Avogadro’s law, one mole of any gas will occupy a volume of approximately 22.4 liters at STP. This holds true because at STP, the behavior of gases is ideally similar and the volume primarily depends on the number of particles (moles), not the type of gas.
Therefore, when assuming the same mass for different gases at STP, the volume will not vary between gases such as hydrogen (H₂) and oxygen (O₂). The difference arises in the amount of moles you get for that mass due to different molecular weights. For example, hydrogen has a molecular weight of approximately 2 g/mol, while oxygen has a molecular weight of about 32 g/mol. If you have the same mass of both gases, you will have more moles of hydrogen than oxygen, but if you compare equal moles of each gas, they will occupy the same volume at STP regardless of their molecular weight or formula.