Why one mole of hydrogen molecule and 1 mole of hydrogen atoms have different masses?

Answer 1

1 mol of hydrogen molecule = 1 mol of H2 = molar mass of H2 = 2.0gmol-1

2.0gmol-1 x 1mol = 2.0g

1 mol of hydrogen atom = 1 mol of H = molar mass of H = 1.0gmol-1

1.0gmol-1 x 1mol = 1.0g

Hence, they have different masses

Answer 2

One mole of hydrogen molecules contains 2 moles of hydrogen atoms. Since molecules have more mass due to the additional particle (electron in the case of hydrogen), the mass of one mole of hydrogen molecules is greater than that of one mole of hydrogen atoms. This difference in mass is due to the molecular structure and the number of constituent particles in each.