Question

Aspirin has the formula ${\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4$. How many atoms of oxygen are there in a tablet weighing 360mg?

Answer 1

Step 1: Calculating the moles

The molecular mass of aspirin is $(9\times 12)+(8\times 1)+(16\times 4)=180\mathrm{g}$

$$\begin{gathered} \therefore \mathrm{Number}\mathrm{of}\mathrm{moles}=\frac{\mathrm{Given}\mathrm{weight}}{\mathrm{Molecular}\mathrm{weight}} \\ \mathrm{Number}\mathrm{of}\mathrm{moles}=\frac{360\times {10}^{-3}}{180} \\ \mathrm{Number}\mathrm{of}\mathrm{moles}=2\times {10}^{-3}\mathrm{moles} \end{gathered}$$

Step 2: Calculating the atoms

The total moles of oxygen present in ${\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4$is $4\times 2\times {10}^{-3}\mathrm{moles}$

The number of atoms in 1 mole of oxygen is $6.022\times {10}^{23}$

$\therefore$the number of atoms in $4\times 2\times {10}^{-3}\mathrm{moles}$ of oxygen is $4\times 2\times {10}^{-3}\times 6.022\times {10}^{23}=4.81\times {10}^{21}$

Therefore, the number of atoms are $4.81\times {10}^{21}$.