At STP, 1 liter of H2(g) and 1 liter of He(g) have the same: A) number of atoms B) density C) number of molecules D) mass

According to Avogadro's Law, equal volumes of different
gases at the same temperature and pressure have the same number of constituent particles
or molecules.

Hydrogen gas is found in the form of
molecules consisting of two hydrogen atoms. Helium gas is found as independent helium
atoms. As there is no option for constituent particles, we can take consider helium to
be in the form of molecules with each molecule consisting of one helium
atom.

So we have the number of molecules in one liter of
the two gases at STP being the same. The correct option is C.