How many atoms are there in 2.50 mol of hydrogen?

For this problem you don't need the Atomic Mass of the element. If you want to convert moles to atoms, you need to take the number of moles and multiply it by Avogadro's constant, 6.02 × 1023. Divide by one mole for units to cancel.

2.50 moles H × (6.02 × 1023 atoms) = 1.51 ×1024 atoms H