Finding actual Yield, Theoretical yield and percent yield

Question

A student was synthesizing Aspirin (C9H8O4) in the laboratory produced by the reaction of salicylic acid (C7H6O3) and acetic anhydride (C4H6O3). Using 4.0 g of salicylic acid as the limiting reactant, determine the theoretical yield. When the student weighed the aspirin product on the balance, the mass was 7.44g. a) What is the actual yield of aspirin? b) What is the theoretical yield of aspirin? c) Calculate the percent yield for this synthesis?

Answer 1

Synthesis of aspirin:

Salicylic acid + acetic anhydride ==> aspirin + acetic acid

Theoretical yield of aspirin when salicylic acid (SA) is limiting:

4.0 g SA x 1 mol SA/138 g x 1 mol aspirin/mol SA x 180 g aspirin/mol = 5.22 g aspirin

The value of 7.44 g of aspirin cannot be correct as that would represent a yield greater than 100% which is not possible, unless of course, the weighed product was still wet with water and not properly dried.

Answer 2

In the reaction H2O is another product of the reaction. Not possible to balance the equation as written in the problem.

H2O is the other product.

When added and chemical equation is balanced there are 2 moles of salicylic acid reacted and 2 moles of aspirin produced.

Knowing this info:

Step 1: Calculate moles of salicylic acid reacted: 4 g / 138 g salicylic acid per mole = .029 mole reacted( sig figs)

Step 2: Calculate grams of Aspirin produced .029 mole aspirin X 180 g / mole aspirin = 5.22 g aspirin produced if all of salicylic acid is reacted and all the aspirin is recovered. This is your theoretical yield.

Step 3: Actual yield of aspirin = 7.44 g.

Critical analysis of problem: impossible to have actual yield be greater than the theoretical yield.

There is an incorrect number in this problem , because your percent yield is 7.44 g / 5.22 g = 143 %.

This disobeys the law of conservation of mass. Please check the numbers.

Yours in learning!

Dale