For this problem, to determine the number of moles given the number of atoms, we use the Avogadro's number. This is an empirical value that relates the number of particles (e.g. atoms, molecules, sub-particles) in one mole of any substance. The value is 6.022×10²³ molecules/mol.
3.50 mol NO₂ * 6.022×10²³ molecules/mol = 2.1078×10²⁴ molecules of NO₂
