Question

(a) Write the molecular formula of acetylsalicylic acid (see Fig. 7.34a).

(b) An aspirin tablet contains 325 mgacetylsalicylic acid. Calculate the number of moles of that compound in the tablet.

Short answer

The molecular formula of acetylsalicylic acid is ${\text{C}}_{\text{9}}{\text{H}}_{\text{8}}{\text{O}}_{\text{4}}$. The aspirin tablet contains $1.80\times {10}^{-3}\text{mol}$of acetylsalicylic acid.

Step-by-step solution

a) Molecular formula of acetylsalicylic acid.

The structural formula of acetylsalicylic acid is shown below.

The structural formula of acetylsalicylic acid suggests that it contains nine (9) carbon (C) atoms, eight (8) hydrogen (H) atoms, and four (4) oxygen (O) atoms.

Hence, the molecular formula of acetylsalicylic acid is ${\text{C}}_{\text{9}}{\text{H}}_{\text{8}}{\text{O}}_{\text{4}}$.

b) calculate the number of moles of acetylsalicylic acid in the aspirin tablet.

The molar mass of carbon${\text{(C) = 12.011 g mol}}^{\text{-}}$

The molar mass of hydrogen$\left(\text{H}\right){\text{= 1.0079 g mol}}^{\text{-}}$

The molar mass of oxygen$\left(\text{O}\right){\text{= 15.999 g mol}}^{\text{-}}$

Now, calculate the molar mass of the acetylsalicylic acid.

$$\begin{gathered} =\left(9\times 12.011\mathrm{g}{\mathrm{mol}}^{-}\right)+\left(8\times 1.0079\mathrm{g}{\mathrm{mol}}^{-}\right)+\left(4\times 15.999\mathrm{g}{\mathrm{mol}}^{-}\right) \\ =180.158\mathrm{g}{\mathrm{mol}}^{-} \end{gathered}$$

Mass of acetylsalicylic acid present in tablet = 325 mg

Now calculate the number of moles of acetylsalicylic acid present in the aspirin tablet

$=325\mathrm{mg}{\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4\times \frac{1\mathrm{g}{\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4}{1000\mathrm{mg}{\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4}\times \frac{1\mathrm{mol}{\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4}{180.158\mathrm{g}{\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4}$

$=325\times 0.001\times 0.0055$

$=1.80\times {10}^{-3}\mathrm{mol}{\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{O}}_4$

Hence, the number of moles of acetylsalicylic acid ${\text{(C}}_{\text{9}}{\text{H}}_{\text{8}}{\text{O}}_{\text{4}}\text{)}$present in the aspirin tablet containing 325 mg acetylsalicylic acid is $1.80\times {10}^{-3}\mathrm{mol}$.