Chemistry Questions

Question

1. Calculate the volume of 0.5000 M NaOH that will be required to titrate a 0.1500g sample of

aspirin.

2. A student obtained reached the end point of the titration 0.76 mL prior to the calculated value

in (1). What was the purity of the sample?

3. A student required 0.40 mL more NaOH than she calculated she would need in question 1.

Explain how this might happen.

Answer 1

1) First, we need to determine how many moles of aspirin we have to neutralize. After a quick internet look-up of the structure of aspirin (C9H8O4), the molar mass is approximately 180.16 g/mol. Using that, we can use stoichiometric ratios to determine how many moles of NaOH are needed for every mole of acid. From there, we can use moles of NaOH and the molarity of the titrant (0.5 M) to determine the volume of titrant needed (note, it will be in liters).

2) To determine percent purity, all that is needed is a ratio between the two values. In this case, it would be the 0.76 mL over the value calculated in part 1. This would then be multiplied by 100 to get a percent purity.

3) There are several possibilities as to why more titrant was needed. Two examples can be that the titrant was of a lesser purity than expected (maybe around 0.45 and not 0.5 exactly) or that there was more acid in the sample than expected.