Question

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 NaOH. The reaction of aspirin with NaOH is as follows: \(\mathrm{C}_{9} \mathrm{H}_{8} \mathrm{O}_{4}(s)+2 \mathrm{OH}^{-}(a q)\) $$ \mathrm{C}_{7} \mathrm{H}_{3} \mathrm{O}_{3}^{-}(a q)+\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) $$ A sample of aspirin with a mass of 1.427 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 NaOH. Calculate the purity of the aspirin. What indicator should be used for this titration? Why?

Short answer

The purity of aspirin is approximately 99.56% and the suitable indicator for this titration is phenolphthalein.

Step-by-step solution

Calculate moles of HCl used in titration.

Use the volume and molarity of HCl to calculate the moles of HCl used in the titration. Moles of HCl = Volume of HCl × Molarity of HCl Moles of HCl = 31.92 mL × 0.289 mol/L = 0.00921888 mol

Calculate moles of excess NaOH.

Since 1 mole of HCl reacts with 1 mole of NaOH, the moles of NaOH are equal to the moles of HCl. Moles of excess NaOH = 0.00921888 mol #Step 2: Calculate moles of aspirin reacted with initial NaOH#

Calculate total moles of NaOH used for hydrolysis.

Use the volume and molarity of NaOH to calculate the total moles of NaOH. Moles of NaOH = Volume of NaOH × Molarity of NaOH Moles of NaOH = 50.00 mL × 0.500 mol/L = 0.025 mol

Calculate moles of NaOH reacted with aspirin.

Subtract the moles of excess NaOH from the total moles of NaOH. Moles of NaOH reacted with aspirin = Moles of NaOH - Moles of excess NaOH Moles of NaOH reacted with aspirin = 0.025 mol - 0.00921888 mol = 0.01578112 mol

Calculate moles of aspirin.

For every 2 moles of NaOH reacted, 1 mole of aspirin reacts. Divide the moles of NaOH reacted with aspirin by 2. Moles of aspirin = 0.01578112 mol / 2 = 0.00789056 mol #Step 3: Determine the mass of aspirin reacted and calculate its purity#

Calculate mass of aspirin reacted.

Use the moles of aspirin and its molar mass to calculate the mass of aspirin reacted. Mass of aspirin reacted = Moles of aspirin × Molar mass of aspirin Mass of aspirin reacted = 0.00789056 mol × 180.16 g/mol = 1.42076 g

Calculate purity of aspirin.

Divide the mass of aspirin reacted by the mass of the aspirin sample and multiply by 100 to get the percentage purity of aspirin. Purity of aspirin = (Mass of aspirin reacted / Mass of aspirin sample) × 100 Purity of aspirin = (1.42076 g / 1.427 g) × 100 = 99.56 % #Step 4: Suggest an appropriate indicator and explain the choice#

Determine the appropriate pH indicator.

As the titration involves a strong acid (HCl) and a strong base (NaOH), the equivalence point occurs at a pH of approximately 7. A suitable pH indicator for this titration is phenolphthalein, which has a pH range of 8.2 - 10.0. It will change color near the equivalence point and provide a visual indication of when all the excess NaOH has been neutralized by the HCl. The purity of aspirin is approximately 99.56% and the suitable indicator for this titration is phenolphthalein.

Key concepts

Purity Determination

Determining the purity of a substance involves a process that helps understand how much of the sample is actually the desired compound rather than impurities. In this case, purity determination of aspirin is achieved through a method called titration analysis. It provides insight into the exact proportion of aspirin in the given sample. During a titration for purity determination, the first step is usually to react the compound of interest with a known reagent. By quantifying the remaining reagent after the reaction with the target compound, we can determine how much of the target compound reacted. This information helps in calculating the compound's purity. For aspirin, a weighed sample is hydrolyzed with a known sodium hydroxide \((NaOH)\) solution. After allowing the reaction to proceed, you'll titrate the remaining unreacted NaOH with a solution of hydrochloric acid \((HCl)\). The results from this titration allow us to calculate the initial amount of aspirin present. With this information, we can accurately determine the purity of the aspirin sample.

Aspirin Hydrolysis

Hydrolysis refers to the chemical breakdown of a compound due to reaction with water. In the case of aspirin, hydrolysis involves breaking down into its constituent parts using an alkaline, such as NaOH. This is an essential step in analyzing its composition and purity. When aspirin is mixed with a strong base like NaOH, it reacts with hydroxide ions (OH\(^-\)) leading to the formation of salicylate and acetate ions as products. In this hydrolysis reaction:

• The chemical equation is: \(\mathrm{C}_9\mathrm{H}_8\mathrm{O}_4 + 2\,\mathrm{NaOH} \to \mathrm{C}_7\mathrm{H}_3\mathrm{O}_3^- + \mathrm{C}_2\mathrm{H}_3\mathrm{O}_2^- + 2\,\mathrm{Na}^+ + 2\,\mathrm{H}_2\mathrm{O}\)
• The breaking of the ester bond in aspirin is crucial, as it provides a basis for quantifying the effectiveness of the aspirin's hydrolysis.

Once the reaction is complete, we can proceed to measure the remaining NaOH through titration, giving us a quantitative understanding of how much aspirin was in the original sample.

Acid-Base Reaction

An acid-base reaction is a chemical process that involves the exchange of hydrogen ions (protons) between chemical species. In the context of aspirin analysis, the acid-base reaction comes into play during the titration when determining how much NaOH remained after aspirin was hydrolyzed. Here’s how it works:

• Sodium hydroxide (NaOH) serves as a strong base, whereas hydrochloric acid (HCl) acts as a strong acid.
• During the titration, HCl is slowly added to react with the leftover NaOH in the solution.
• At the equivalence point, the moles of hydrogen ions from HCl will equal the moles of hydroxide ions from NaOH, resulting in a neutralization where water is formed.

The titration continues until all the excess base is neutralized, signifying that the acid and base have reacted fully. By knowing the volume and concentration of the acid added, it is possible to calculate how much unreacted NaOH was initially present, which in turn aids in determining the initial amount of aspirin.

Indicator Choice

Selecting the correct indicator is crucial for the success of a titration, especially considering the pH range where the equivalence point is expected to occur. The choice of indicator can significantly influence the accuracy of the titration endpoint. In our context, where we have a titration between a strong base (NaOH) and a strong acid (HCl), the equivalence point will occur around a neutral pH of 7. To choose the right indicator:

• Phenolphthalein is often recommended in this case because it changes color in the pH range of approximately 8.2 to 10.0. Even though the equivalence point is actually closer to 7, phenolphthalein's distinct color change ensures a reliable detection just past the equivalence point, simplifying the observation process.
• A vivid transition from colorless to pink (or vice versa) effectively signals when all the excess NaOH has been neutralized by the HCl.

Choosing the right indicator not only makes the endpoint determination easier but also ensures that experimental data, such as percentage purity calculated, is reliable and accurate.