Question

One method for determining the purity of aspirin (empirical formula C₉H₈O₄) is to hydrolyze it with NaOHsolution and then to titrate the remaining NaOH. Thereaction of aspirin with NaOH is as follows:

$$\begin{gathered} {\mathbf{C}}_{\mathbf{9}}{\mathbf{H}}_{\mathbf{8}}{\mathbf{O}}_{\mathbf{4}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathbf{+}\mathbf{2}{\mathbf{OH}}^{\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{\to } \\ {\mathbf{C}}_{\mathbf{7}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{O}}_{\mathbf{3}}^{\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}{\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{3}}{\mathbf{O}}_{\mathbf{2}}^{\mathbf{-}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{+}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{l}\mathbf{\right)} \end{gathered}$$
A sample of aspirin with a mass of 1.427 g was boiledin 50.00 mL of 0.500 M NaOH. After the solution wascooled, it took 31.92 mL of 0.289 M HCl to titrate theexcess NaOH. Calculate the purity of the aspirin. Whatindicator should be used for this titration? Why?

Short answer

Theindicator is bromothymol blue and purity of aspirin is 99.5%.

Step-by-step solution

Given Information

The mass of aspirin is 1.427 g. The boiling of aspirin in 50 mL of 0.5 M NaOH.

Number of moles of adding and left NaOH

The number of moles of NaOH added are$\frac{0.5}{1000 \mathrm{mL}}\times 50 \mathrm{mL}=0.025 \mathrm{mol}$ The number of moles of NaOH unreacted are $\frac{0.289}{1000 \mathrm{mL}}\times 31.92 \mathrm{mL}\times \frac{1 \mathrm{mol}}{1 \mathrm{mol}}=0.0092 \mathrm{mol}.$The number of moles of NaOH reacted with aspirin are 0.025-0.0092=0.0158 mol. The presence of aspirin mass is $0158 \mathrm{mol}\times \frac{1 \mathrm{mol}}{1 \mathrm{mol}}\times 180.2 \mathrm{g}/\mathrm{mol}=1.42 \mathrm{g}$The purity can be found as $\frac{1.42 g}{1.427 g}\times 100=99.5\%.$The strong acid base titration can be done and pH is 7. The indicator can be found as bromothymol blue as this indicator changes color from base to acid around ~pH 7.