A student weighed out 1.5g of salicylic acid into a flask and then added 4ml of acetic an hydride to react. He also added 5 drops of concentrated sulfuric acid. Density of acetic an hydride is 1.08g/ml and sulfuric acid is 1.85g/ml (sulfuric acid is a catalyst in this reaction). What is the limiting reagent in this reaction? and assuming the reaction went to completion how much product should be expected to be produced (theoretical yield)?

Answer 1

Have you written the equation? It is a 1:1 reaction (1 mole SA and 1 mole acetic anhydride produces 1 mole aspirin).
Convert 1.5 g SA to moles. mols = grams/molar mass.
Convert 4 mL acetic anhydride to grams (using density), then to moles.

Convert moles SA to moles product (aspirin).
Convert moles acetic anhydride to moles product.
It is quite likely that the two answer for moles of the product will not be the same which means one of them is wrong. The correct answer, in limiting reagent problems, is ALWAYS the smaller one and the reagent producing the smaller value is the limiting reagent.

Now convert moles of the product to grams. g = moles x molar mass. This is the theoretical yield.