Two quick Chemistry questions. Thanks !

Question

Salicyclic acid + acetic anhydrade ---> acetyl salicyclic acid + acetic acid

1. In the above reaction, if 2.0 g of salicylic acid and 3.0 g of acetic anhydride were used. What is the limiting reagent and what is the theoretical yield of acetyl salicylic acid? (molar masses: C (12.0 g mol^-1); H (1.008 g mol-1); O (16.0 g mol^-1)
2. If you react 1.2g of salicylic acid with excess ethanolic anhydride and a catalytic amount acid and obtain 1.34 g of product, what is the percentage yield?

Thank you guys.

Answer 1

1) Theoretical yield

For the chemical reaction, I am going to use the following abbreviations:

SA = Salicylic acid

Ac₂O = Acetic anhydride

ASA = Acetylsalicylic acid

Convert grams of SA and Ac₂O to grams of ASA. The reactant that produces the lower amount of ASA is the limiting reagent AND, the mass of ASA formed is the theoretical yield. Note that in the chemical reaction, the mole ratio between the 2 reactants and the product is 1:1.

2.0 g SA x (1 mol SA / 138.12 g SA) x (1 mol ASA / 1 mol SA) x (180.16 g ASA / 1 mol ASA) = 2.6 g ASA

3.0 g x (1 mol Ac₂O / 102.09 g Ac₂O) x (1 mol ASA / 1 mol Ac₂O) x (180.16 g ASA / 1 mol ASA) = 5.3 g ASA

As you can see, SA produces less ASA, therefore, it is the limiting reagent, and the theoretical yield = 2.6 g

2) Percentage yield

Calculate the theoretical yield of ASA starting with 1.2 g of SA.

1.2 g SA x (1 mol SA / 138.12 g SA) x (1 mol ASA / 1 mol SA) x (180.16 g ASA / 1 mol ASA) = 1.6 g ASA

%-Yield = (actual yield / theoretical yield) x 100%

= (1.34 / 1.6) x 100% = 84%