Which of the following samples contains the greatest number of atoms?

Explanation

Every sample contains one mole of molecules, so the answer depends on the number of atoms in each molecule: helium has one, oxygen two, water three and ammonia four. One mole of ammonia therefore contains 4 x 6.02 x 10^23 atoms, more than any of the others. Confusing moles of molecules with moles of atoms is exactly the error this question is designed to expose.