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

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justaguide | College Teacher | (Level 2) Distinguished Educator

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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.