One method for determining the purity of aspirin (C₉H₈O₄) is to hydrolyze it with NaOH solution and then to titrate the remaining NaOH. The reaction of aspirin with NaOH is as follows:

A sample of aspirin with a mass of 1.427 g was boiled in 50.00 mL of 0.500 M NaOH. After the solution was cooled, it took 31.92 mL of 0.289 M HCl to titrate the excess NaOH. Calculate the purity of the aspirin. What indicator should be used for this titration? Why?

Interpretation: One of the methods for determining the purity of aspirin is to hydrolyze it with NaOH solution and then titrating the remaining NaOH . The reaction of aspirin with NaOH is given. The purity of the given amount of aspirin and the indicator suitable to be used is to be determined.

Concept introduction: A technique in which the concentration of an unknown solution is calculated by using a solution of a known concentration is known as titration.

A substance that depicts a change in color corresponding to a chemical change in a solution is known as an indicator.

To determine: The purity of the given amount of aspirin and the indicator suitable to be used.

Explanation

To find the number of moles of aspirin and  number of moles of NaOH

The given mass of aspirin is 1.427 g .

The volume of NaOH is 50.0 mL (0.050 L) .

The volume of HCl is 31.92 mL (0.03192 L) .

The molarity of NaOH is 0.500 M .

The molarity of HCl is 0.289 M .

The molar mass of aspirin is 180 g/mol .

The stated chemical reaction is,

C9H8O4(s)+2OH−(aq)→C7H5O3−(aq)+C2H3O2−(aq)+H2O(l)

The molar mass is calculated by the formula,

Number of moles=Given massMolar mass

Substitute the value of the given mass of aspirin and its molar mass in the above expression.

Number of moles=1.427 g180 g/mol=7.92×10-3 mol_

The number of moles is also calculated by the formula,

Number of moles=Volume (L)×Molarity

Substitute the value of the volume and molarity in the above expression.

For NaOH ,

Number of moles=0.050 L×0.500 M=0.025 mol_

For HCl ,

Number of moles=0.03192 L×0.289 M=9.2×10-3 mol_

To find the moles of aspirin needed

The HCl reacts with the excess hydroxide present in the solution