Though more potentially dangerous, a given volume of hydrogen gas will lift more weight than an equal volume of helium because hydrogen......

Though more potentially dangerous, a given volume of hydrogen gas will lift more weight than an equal volume of helium because hydrogen is less dense than helium.

Use Avogadro's hypothesis to explain why hydrogen is less dense than helium.

If you have one mole of each, and since hydrogen has less mass per mole than helium, it makes hydrogen less dense than helium.