Question

In lab, aspirin was made from salicylic acid through a process of heating water to 85 degrees celcius, recrylstalize doing a vacuum filtrtion apparatus with both cooled and warm filtration. After heating in an oven for one hour and letting cool. The mass signficatnly decreased after heating to be about .0129 grams aspirin- the starting SA was 1.4460 why was there such a decrease? The melting point was tested using a melting point capillary with a Mel-Temp apparatus. The melting point that was found was a range from 150-155degrees celcius which is not the same as the melting point of asprin. What could be a potential cause for this error? The asprin also failed the indicator test with the cresol red comparing the SA to the ASA which both turned purple. What is a cause for these results?

Answer 1

Solution:

Aspirin is synthesized in the laboratory by the reaction of salicylic acid (SA) with acetic anhydride. The products of the reactions are acetyl salicylic (ASA) acid and acetic acid as given by the following reaction:

Ans to part A:

In chemical calculations, it is always assumed that the reaction goes to completion. However, in reality this seldom happens and the expected amount of product is not usually obtained. Percentage yield is calculated by the using the formula:

Percent yield = (Actual yield/Theoretical yield)x 100

There can be many reasons for less yield like:

Transferring the reaction mixture will cause some product to be lost.

Reaction may not be entirely complete because of (a) not waiting long enough (b) not mixing the reactants properly (c) the product may contain impurities (d) Another source of product loss was during the filtration of the product. The aspirin was small, powdery crystals. Since they were so small, some could have passed through the filter paper.

Because of all these reasons although 1.4460 g of salicylic acid (which is the limiting reagent in this case) the yield of aspirin was 0.0129 g.

Ans to Part B: The melting of pure aspirin is 135- 137 °C. A pure organic compound usually melts over a range of two degrees or less. A sample is impure if it has a melting point range that is lower and/or wider than that the literature value. More impurities increase this effect. In the present case the difference in melting point is around 20 °C which is very high and is attributed to the presence of high amount of impurities in the aspirin product.

Ans to Part C: The failure of color test with the indicator is that the product aspirin is contaminated with salicylic acid. Another test with ferric chloride can be performed. If salicylic acid is added to a solution of ferric chloride, it will turn the color of ferric chloride to purple whereas a pure sample of aspirin will remain yellow.