Chemistry and Chemical Reactivity (9th Edition)

Molar mass of $O_2: 31.998\ g/mol$, Avogadro's number: $6.022\times10^{23}\ molecules/mol$ In 16 g of $O_2$: $16\ g\div31.998\ g/mol\times6.022\times10^{23}\ molecules/mol=3.011\times10^{23} molecules$ 2 atoms of O/molecule of $O_2$ $6.022\times10^{23}\ atoms$ of O.