In which of the following pairs do the two samples contain the same total number of atoms? ($\mathrm{H}=1$, $\mathrm{C}=12$, $\mathrm{N}=14$, $\mathrm{O}=16$; 22.4 L/mol at STP)

(a) 4 g H2 = 2 mol x 2 = 4 mol atoms; 64 g O2 = 2 mol x 2 = 4 mol atoms (equal). (b) 0.5 mol He versus 0.5 mol x 5 = 2.5 mol atoms in CH4 (unequal). (c) 18 g water = 1 mol x 3 = 3 mol atoms; 33.6 L O2 = 1.5 x 2 = 3 mol atoms (equal). (d) 34 g NH3 = 2 x 4 = 8 mol atoms versus 1 x 3 = 3 mol atoms (unequal).