One method for determining the purity of aspirin \(\left(\mathrm{C}_{9} \mathrm{H}_{8} \mathrm{O}_{4}\right)\) is to hydrolyze it with NaOH solution and then to titrate the remaining \(\mathrm{NaOH}\). The reaction of aspirin with \(\mathrm{NaOH}\) is as follows: \(\mathrm{C}_{?} \mathrm{H}_{3} \mathrm{O}_{4}(s)+2 \mathrm{OH}^{-}(a q)\) Aspirin $$ \begin{array}{c} \text { Bail } \mathrm{C}_{7} \mathrm{H}_{5} \mathrm{O}_{3}^{-}(a q)+\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \\ \text { Salicylate ion Acetate ion } \end{array} $$ A sample of aspirin with a mass of \(1.427 \mathrm{~g}\) was boiled in \(50.00 \mathrm{~mL}\) of \(0.500 M \mathrm{NaOH}\). After the solution was cooled, it took \(31.92 \mathrm{~mL}\) of \(0.289 \mathrm{M} \mathrm{HCl}\) to titrate the excess \(\mathrm{NaOH}\). Calculate the purity of the aspirin. What indicator should be used for this titration? Why?

Step by step solution

01

Calculate the moles of NaOH initially present

First, find the number of moles of NaOH initially present in the 50.00 mL solution. We can use the formula moles = volume × concentration. Initial moles of NaOH = volume of NaOH × concentration of NaOH = \(50.00 \, mL \times 0.500 \, M\) = 0.025 moles

02

Calculate the moles of HCl used in the titration

Next, find the number of moles of HCl used to titrate the excess NaOH. We can use the same formula as in step 1. Moles of HCl = volume of HCl × concentration of HCl = \(31.92 \, mL \times 0.289 \, M\) = 0.00922248 moles

03

Calculate the moles of NaOH leftover after reacting with aspirin

Now, we will find the moles of NaOH remaining in excess after the reaction with aspirin. This can be done by using the stoichiometry of the reaction between the excess NaOH and the HCl used in the titration (1:1 ratio). Moles of leftover NaOH = moles of HCl = 0.00922248 moles

04

Calculate the moles of NaOH that reacted with aspirin

Subtract the moles of leftover NaOH from the initial moles of NaOH to determine the moles of NaOH that reacted with the aspirin. Moles of NaOH that reacted with aspirin = initial moles of NaOH - moles of leftover NaOH = 0.025 moles - 0.00922248 moles = 0.01577752 moles

05

Calculate the moles of aspirin

Now, using the stoichiometry of the reaction between aspirin and NaOH (2:1 ratio), find the number of moles of aspirin. Moles of aspirin = 0.5 × moles of NaOH that reacted with aspirin = 0.5 × 0.01577752 moles = 0.00788876 moles

06

Calculate the mass of pure aspirin in the sample

To find the mass of pure aspirin in the sample, multiply the moles of aspirin by the molar mass of aspirin (180.16 g/mol). Mass of pure aspirin = moles of aspirin × molar mass of aspirin = 0.00788876 moles × 180.16 g/mol = 1.42061 g

07

Calculate the purity of the aspirin

Now, divide the mass of pure aspirin by the mass of the aspirin sample and express the result as a percentage to find the purity of the aspirin. Purity of aspirin = (mass of pure aspirin ÷ mass of aspirin sample) × 100 = \(\frac{1.42061 \, g}{1.427 \, g} \times 100\) = 99.55%

08

Identify a suitable indicator and justify its use

A suitable indicator for this titration is phenolphthalein because it changes its color near the pH of the equivalence point (pH ≈ 7) between a strong base (NaOH) and a strong acid (HCl). The purity of the aspirin sample is 99.55%, and phenolphthalein is a suitable indicator for this titration due to its color change near the pH of the equivalence point between NaOH and HCl.

Unlock Step-by-Step Solutions & Ace Your Exams!

• Full Textbook Solutions

  Get detailed explanations and key concepts

• Unlimited Al creation

  Al flashcards, explanations, exams and more...

• Ads-free access

  To over 500 millions flashcards

• Money-back guarantee

  We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!