A 12.8 g sample of NaOH contains how many molecules?

To determine the number of molecules present for a given mass of a substance, we need to convert it from units of mass to moles by using the molar mass. Then, we use Avogadro's number to convert it to moles. We do as follows:

12.8 g NaOH (1 mol NaOH / 40 g NaOH) (6.022 x10^23 molecules NaOH / 1 mol NaOH) = 1.927x10^23 molecules NaOH present