Question

Calculate the Theoretical Yield and Percent Yield for the reaction of p-Amino Benzoic Acid (PABA) + Ethanol + Sulfuric Acid to create Benzocaine. Is Sulfuric Acid simply a catalyst in this reaction? 

Molar Mass Benzocaine: 165.19g/mol

Molar Mass PABA: 139.14g/mol

Molar Mass Ethanol: 46.07g/mol

Molar Mass of Sulfuric Acid: 98.079g/mol

Mass of PABA Used: 0.5093g (0.00366 moles)

Volume of Ethanol Used: 7.0 mL (5.523g) (0.11986 moles) Ethanol is in excess here.

Volume of Sulfuric Acid Used: 0.50 mL (0.92g) (0.00938 moles)

Mass of Benzocaine collected: 0.3722g

Answer 1

 

Moles of PABA used = 0.00366 mol

Moles of ethanol used = 0.11986 mol

Moles of sulfuric acid = 0.00938 mol

Mass of benzocaine collected = 0.3722 g

Molar mass of benzocaine = 165.19 g/mol

Moles of benzocaine = mass of benzocaine/molar mass of benzocaine

                                  = 0.3722/165.19

                                  = 0.0023 mol

1 mol of PABA reacts with 1 mol of ethanol to produce 1 mol of benzocaine.

Moles of PABA is 0.00366 which is the least among the reagents. Therefore, PABA is the limiting reagents.

Therefore theoretically, 0.00366 moles of PABA should produce 0.00366 moles of benzocaine.

Therefore theoretical yield = 0.00366 x 165.19 = 0.6046 g

Percent yield = (actual yield x 100)/theoretical yield

                     = 0.3722 x 100/0.6046

                     = 61.56 %

Sulfuric acid acts as a catalyst in Fischer esterification between PABA and ethanol.

In this reaction sulfuric acid is used 0.00938 mol which is greater than the moles of PABA (0.00366 mol) in this reaction. Sulfuric acid is used in excess because some of it will get consumed by the amine group on PABA and also sulfuric acid acts as dehydrating agent as it will soak up the water produced in the esterification reaction and thus shifting the equilibrium of the reaction towards product side.