Question

What is the % yield of the reaction making 2.7 g of acetyl salicylic acid from 6.4 g of salicylic acid, 5.6 mL of acetic anhydride (density 1.082), and 10 drops 85 % weight phosphoric acid (density 1.685)? Use the molecular weights that you have already calculated. Give two significant figures.

Answer 1

Ans. The balanced reaction for synthesis of acetylsalicylic acid (aspirin) is as follow-

C₇H₆O₃ + C₄H₆O₃ ------> C₉H₈O₄ + CH₃COOH             - reaction 1

C₇H₆O₃ = salicylic acid

C₄H₆O₃= acetic anhydride

C₉H₈O₄ = aspirin

CH₃COOH = Acetic acid

Theoretical molar ratio of reactants = Salicylic acid : Acetic anhydride = 1 : 1

# Determine the limiting reactant, if any-

Moles of salicylic acid = Mass / Molar mass = 6.4 g / (138.12 g/ mol) = 0.046337 mol

Moles of acetic anhydride = (5.6 mL x 1.082 g mL^-1) / (102.09 g/ mol) = 0.059352 mol

Note that moles of salicylic acid is lesser than that of acetic anhydride. So, salicylic acid is the limiting reactant following the 1:1 theoretical molar ration of reactants.

# The stoichiometry of product follows the stoichiometry of limiting reactant.

According to stoichiometry of balanced reaction, 1 mol salicylic acid produces 1 mol aspirin. So, theoretically, 1 mol salicylic acid shall produce 1 mol acetyl salicylic acid (ASA)

So,

Theoretical moles of ASA produced = 0.046337 mol = Moles of salicylic acid

Theoretical mass of ASA produced = Moles x Molar mass

                                                            = 0.046337 mol x (180.16 g/ mol)

                                                            =8.348 g

Therefore, theoretical yield of reaction = 8.348 g

# % yield = (Actual yield / Theoretical yield) x 100

                        = (2.7 g / 8.348 g) x 100

                        = 32.34 %

                        = 32 %