Answer
Approximately 1.672 $\times$ $10^{21}$ molecules of aspirin.
Work Step by Step
As we found in 3.90a, there are 0.002778 mols of aspirin in one 0.5g tablet.
With that information, we can use this formula:
Number of molecules = Number of mols $\times$ 6.02 $\times 10^{23}$
Number of molecules = 0.002778 $\times$ 6.02 $\times 10^{23}$
Number of molecules = 0.01672 $\times 10^{23}$
Number of molecules = 1.672 $\times 10^{21}$ molecules.
