How many atoms are in 2.50 mol of hydrogen?

To do this, you must multiply the number of moles by the Avogadro's number, 6.0221409e+23

So, with 2.50 mol,

2.50 mol H * 6.02 * 10^23 atoms H / 1 mol H = 1.51 * 10^24 atoms H.

To make that statement clearer, in 2.50 mol of Hydrogen, there are 1.51x10^24 atomsH