In this case, the hydrogen atoms are present in the (NH3)2 part of the molecule. The sign (NH3)2 mean that there are two NH3 molecules in their compound. That means the total atom of hydrogen would be: 3*2 = 6 atom for every compound molecule
Since there is 0.7 mole of compound, then the hydrogen atom would be:
0.7 mole * 6= 4.2 moles
