How many hydrogen atoms are in 4.00 mol of ammonium sulfide?

1.93 x 10^25

The formula for ammonium sulfide is (NH4) 2S, so you can determine that there's 8 hydrogen atoms per molecule of ammonium sulfide. So just multiply the number of moles you have by 8 giving 4.00 * 8 = 32.00 moles. Then to get the number of actual atoms, just multiply again by avogadro's number.

32.00 * 6.0221409 x 10^23 = 1.93 x 10^25