How many molecules of aspirin (formula mass 180amu) are in a 0.500g sample?

First let us calculate the number of moles:

number of moles = 0.500 g / (180 g / mol) = 2.78 x 10^-3 mol

From Avogadros number, we know that there are 6.022 x 10^23 molecules per mole, therefore:

number of molecules = 2.78 x 10^-3 mol * (6.022 x 10^23 molecules / mole)

= 1.67 x 10^21 molecules