Question

Aspirin or acetyl salicylic acid is synthesized by reacting salicylic acid with acetic anhydride: C7H6O3 (salicylic aci...


Aspirin or acetyl salicylic acid is synthesized by reacting salicylic acid with acetic anhydride:

C7H6O3 (salicylic acid) + C4H6O3 (acetic anhydride) = C9H8O4 (aspirin) + C2H4O2 (acetic acid)

(a) How much salicylic acid is required to produce 0.400 g of aspirin (about the content in a tablet), assuming the acetic anhydride is present in excess?

(b) Calculate the amount of salicylic acid needed if 74.9% of salicylic acid is converted to aspirin.

(c) In one experiment, 9.26 g of salicylic acid is reacted with 8.54 g of acetic anhydride. Calculate the theoretical yield of aspirin and the percent yield if only 10.9 g of aspirin is produced.

Answer

Since the given reaction is

C7H6O3 + C4H6O3 -----> C9H8O4 + C2H4O2 

a) Hence from the above balanced reaction, we can say that 1 mole of salicylic acid is required to produce 1 mole of aspirin if 100 % of salicylic acid is converted

Since moles = mass / molar mass 

And molar mass of aspirin = 12 X 9 + 8 X 1 + 4 X 16 = 12 X Atomic mass of C + 8 X Atomic mass of H + 4 X Atomic mass of O = 180 g/mol 

And molar mass of salicylic acid = 7 X Atomic mass of C + 6 X Atomic mass of H + 3 X Atomic mass of O = 138 g/mol

Hence moles of aspirin produced = 4 / 180 = 0.02222 

Since moles of salicylic acid reacted = moles of aspirin produced = 0.02222

Hence mass of salicylic acid = moles X molar mass = 0.02222 X 138 = 3.067 g

b) Since when 100 % of salicylic acid was reacting, we needed 3.067 g of salicylic acid 

Hence if 74.9 % of salicylic acid is converted, the mass required = 3.067 X 100 / 74.9 = 4.095 g

c) Since moles of salicylic acid reacting = mass / molar mass = 9.26 / 138 = 0.0671

And since molar mass of acetic anhydride = 12 X Atomic mass of C +  6 X Atomic mass of H + 3 X Atomic mass of O = 102 g/mol

Hence moles of acetic anhydride reacting = 8.54 / 102 = 0.0837

Since 1 mole of salicylic acid needs 1 mole of acetic anhydride for complete reaction

And since moles of acetic acid is more than required 

Hence salicylic acid is limiting reagent 

Hence moles of aspirin produced theoretically = moles of salicylic acid = 0.0671 

Hence mass of aspirin produced theoretically = moles X molar mass = 0.0671 X 180 = 12.08 g = the theoretical yield of aspirin

Hence percent error = aspirin produced X 100 / the theoretical yield of aspirin = 10.9 X 100 / 12.08 = 90.24 % approx 

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