Standardization of NaOH with benzoic acid and titration of vinegar

This is intended to be part teacher demonstration of proper technique and for students to learn by doing. They may or may not have learned these skills before. I have done this with my Year 1 IB Chemistry students, many of whom had never prepared a standard solution or done a titration before. So I demonstrated the use of each piece of equipment and made them take notes on it before beginning. This took about half an hour.

The aim is that students will learn:

• to make standard solutions
• that water is not always the solvent
• how to carry out a titration
• the importance of standardization
• how to use volumetric flasks, pipettes and burettes to make accurate volume measurements

The instructions are deliberately sparse. This is so that students will figure out what questions to ask and possibly make mistakes and learn from them. This does require a little more supervision from the teacher, but with demonstrations before each step it should not be problematic.

The use of ethanol might be an issue, since each group needs 100ml of it. I tried using ethanol-water mixtures and settled on about 70% ethanol. Lower concentrations seemed to cause benzoic acid to precipitate in pipettes. So the “ethanol” I provided was actually about 70% ethanol. The main reason to use benzoic acid was that we don’t have any potassium hydrogen phthalate, and it was a nice opportunity to use it in practice. 50ml would be fine too, but we don’t have 50 ml volumetric flasks.

The answers to the questions should be easy to figure out and are not provided. The titres should be about 13 ml of NaOH for the reaction with benzoic acid and about 16ml NaOH for the vinegar titration (but this depends on your vinegar).