N3.
DETERMINATION OF PURITY OF AN ORGANIC ACID
Purpose: To determine the purity of a sample of an organic acid
PROCEDURE
1. Obtain an organic acid sample, and note its identity (name, formula, reaction ratio with
NaOH)
2. Weigh accurately about 0.2 – 0.25g (in triplicate) of the acid into a 250 mL conical flask
3. Add about 50 mL of purified water to each flask to dissolve the acid. (Gentle warming
may be required for dissolution)
4. Add 1-2 drops of phenolphthalein and titrate to endpoint with standard NaOH
CALCULATIONS
As part of this exercise, you are required to design an Excel spreadsheet which performs the
calculations. This must be done before leaving the laboratory.
1.
2.
3.
4.
5.
Calculate the moles of NaOH used in the titration
Using the reaction ratio, determine the moles of acid involved in the titration
Calculate the mass of acid involved in the titration
Calculate the % purity of the sample
Average the individual results
QUESTIONS
1. Explain why stronger organic acids are more likely to be soluble in water
2. Would it matter to the analysis results if the acid was insoluble in water? Explain your
answer.
3. Explain why phenolphthalein was used as the indicator
4. Describe how you could determine the identity of an organic acid by titration, assuming
that the sample was 100% pure.