Question: how many molecules are present in 4.21 moles of hydrogen gas?
Answer: Answer: 1 mole of anything contains 6.02 x 1023 particles. (Note that "mol" is the abbreviation for mole.) So, 4.21 mol of hydrogen gas contains 4.21 x 6.02 x 1023 = 2.53 x 1024 molecules of hydrogen.