I saw a question that asked which expression can be used to designate 2 moles of atoms. The choices were

  1. H2
  2. C02
  3. 2N20
  4. 2NaCl
  5. H2SO4

The answer is H2 but I was confused because I thought H2 is only 1 mole because H2 is diatomic. What am I not understanding?

  • 2
    $\begingroup$ Well, hydrogen gas has twice as many atoms as molecules, so any given amount of hydrogen gas has twice as many moles of atoms as moles of molecules. $\endgroup$ – Tanner Swett Jul 15 '16 at 23:59

The key word is 'atom.' H2 has two atoms, so a mole of H2 would have 1 mole of H2 molecules, and when broken down into atoms, 2 moles of atoms.

Think about it like pairs of shoes. If you have 6 pairs of shoes, you have 12 shoes.

CO2 has 3 atoms, 2N20 has 3 atoms and there are already 2 moles so that would be 6 moles of atoms, 2NaCl has 2 atoms but again that 2 indicates that you'd have 4 moles of atoms, and H2SO4 has 7 atoms.


It depends on the exact phrasing of the question, but you seemed to indicate that they were asking "Which of these substances is comprised of two moles of ATOMS?" and NOT "Which of these substances is comprised of two moles of ELEMENTS?"

As hydrogen gas naturally occurs in a diatomic state, it is true that there is only one mole of naturally-occurring elemental hydrogen in answer one, but one mole of H2 is formed from 2 moles of H.

Just to check that we are correct, let's go through all of the answer choices:

  1. H2 - There are 2 moles of hydrogen atoms (1 mole of elemental hydrogen). Total: 2 moles of atoms
  2. There are 1 mole of Carbon atoms and 2 moles of oxygen atoms (oxygen is also naturally diatomic, but we're counting atoms, not elemental configuration). Total: 3 moles of atoms
  3. There are 4 moles of nitrogen atoms (N2, like H2 and O2 is diatomic) and 2 of oxygen atoms. Total: 6 moles of atoms
  4. There are 2 moles of sodium ions and 2 of chlorine ions (you could technically point out that this answer choice doesn't even involve true atoms, but I imagine that the nuance is lost on whoever wrote the question). Total: 4 moles of ions
  5. There are 2 moles of hydrogen, 1 of sulfur, and 4 of oxygen. Total: 7 moles

According to this analysis, answer 1 (H2) is the only substance listed that is composed of two moles of atoms.


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