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 \(\mathrm{NaOH}\). 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?

Short answer

The purity of the aspirin sample is approximately 99.55%. Phenolphthalein is a suitable indicator for this titration because it changes color near the pH of the equivalence point between a strong base (NaOH) and a strong acid (HCl).

Step-by-step solution

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

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

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

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

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

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

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%

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.

Key concepts

Aspirin Analysis

Aspirin, also known as acetylsalicylic acid, is a widely used pharmaceutical product. It requires strict purity standards for effectiveness and safety. Purity determination of aspirin involves quantitative analysis to identify the amount of active ingredient in a given sample.

The typical method for analyzing aspirin involves a chemical reaction with sodium hydroxide (NaOH). This reaction results in the breakdown of aspirin into simpler components that can be quantitatively measured. You first dissolve aspirin in a known concentration of NaOH solution. After the reaction, the excess NaOH is titrated to determine how much of it reacted with the aspirin.

• The aspirin reacts with NaOH, using up some of the base.
• The reaction breaks down aspirin into salicylate and acetate ions, which are not quantified in the purity test.
• The amount of NaOH that reacts provides a means to calculate the amount of aspirin initially present.

By comparing the mass of the pure aspirin calculated from the reaction to the initial mass, you can determine the purity percentage.

Titration

Titration is a classic laboratory method used to determine the concentration of a dissolved substance by reacting it with a reagent of known concentration. In the case of aspirin analysis, titration helps quantify excess NaOH.

You perform the titration after the reaction between aspirin and NaOH. This involves gradually adding a known concentration of hydrochloric acid (HCl) to the solution. The HCl neutralizes the leftover NaOH.

• The point where all leftover NaOH is neutralized is called the equivalence point.
• By measuring the volume of HCl used, you can determine how much NaOH was initially left over.

This step is crucial because by knowing the leftover NaOH, you can calculate how much NaOH reacted with aspirin. This is a key piece of data needed for the purity calculation.

Stoichiometry

Stoichiometry plays a foundational role in calculating the purity of aspirin. It involves the use of balanced chemical equations to determine relationships between reactants and products in a chemical reaction.

In the aspirin analysis scenario, stoichiometry helps establish the quantitative link between the amount of aspirin and NaOH involved in the reaction. The balanced chemical equation shows that two moles of NaOH react with one mole of aspirin.

• First, calculate the initial moles of NaOH using its concentration and volume.
• Subtract the moles of NaOH left unreacted, determined through titration with HCl.
• The difference gives the moles of NaOH that reacted with aspirin.
• Since two moles of NaOH are needed for one mole of aspirin, divide the moles of reacted NaOH by two to find the moles of aspirin.

Finally, converting the moles of aspirin to mass, and comparing to the initial sample mass, allows you to calculate the aspirin's purity.

Acid-Base Indicators

Acid-base indicators are essential in titration since they signal the reaction's equivalence point by changing color. Choosing the appropriate indicator improves the accuracy of the titration.

Phenolphthalein is often used in the titration of NaOH and HCl. Although both are strong, phenolphthalein is ideal because it changes from colorless to pink around the equivalence point, typically near neutral pH.

• The color change informs you when the acid has completely reacted with the base.
• This allows for precise measurement of how much titrant (HCl) was needed.
• Picking an unsuitable indicator would result in inaccurate purity calculations for aspirin.

Thus, understanding the behavior of indicators like phenolphthalein is vital to obtaining reliable results in aspirin purity analysis.