Stoichiometry calculation of Theoretical Yield and Mass Percentage Yield of Acetylsalicylic Synthesis

June 20th, 2008

Stoichiometry calculation of acetyl salicylic acid is easier we solve if we know the complete reaction that involve. The synthesis of acetyl salicylic acid (ASA) is categorized as an esterification of salicylic acid and acetic anhydride that yielding acetyl salicylic acid and acetic acid as by product, a few drops of sulfuric acid often added as a catalyst. This technique is usually employed in undergraduate teaching labs.

Now examine this problem on how you can calculate the theoretical yield of acetylsalicylic synthesis.

“Chemistry student reacts 4.81 g of salicylic acid with an excess of acetic anhydride in the presence of a few drops of sulfuric acid to compose acetylsalicylic acid (aspirin), which the student then purifies. (1) Calculate the theoretical yield mass of product (2) if the student only get aspirin 5,781 g calculate the %yield of the student result ”

Free Chemistry Tutor

In the synthesis of aspirin we usually used excess acetic anhydride; this should be done to ensure that the salicylic acid will be completely reacted and we get the maximum result. But unfortunately there are not chemical reactions that 100% complete, many factors influence them. As the result, we have the theoretical result and % yield of the reaction. Let’s we solve the problem.

The balanced reaction :

+ (CH3CO)2O -> + CH3COOH

Firstly calculate the molecular mass of the salicylic acid and the acetylsalicylic acid (aspirin)

Mr salicylic acid (C7H6O3)
= (7 x 12.01) + (6 x 1.008) + (3 x 16.00)
= 138.118 g/mol

Mr acetylsalicylic acid (C9H8O4)
= (9 x 12.01) + (8 x 1.008) + (4 x 16.00)
= 180.154 g/mol

Secondly calculate the moles of salicylic acid

Moles salicylic acid (C7H6O3)
= mass / molar mass
= 4.81 / 138.118
= 0.035 mol

the mole ratio of salicylic acid and acetylsalicylic acid (aspirin) is 1:1 thus from this stoichiometry we can calculate the moles of acetylsalicylic acid (aspirin) that produced.

Moles of acetylsalicylic acid (aspirin)
= 1/1 x moles of salicylic acid (C7H6O3)
= 1/1 x 0.035 mol
= 0.035 mol

The theoretical mass of acetic salicylic acid (aspirin) obtains by multiplying the moles of aspirin with its molecular mass.

Theoretical mass of acetylsalicylic acid (C9H8O4)
= moles x molar mass
= 0.035 mol x 180.154 g/mol
= 6.274 g

The mass of percentage yield calculated by dividing actual mass of the aspirin that the student got with the theoretical that we solve above in the percent, and we have

The % yield of the aspirin
= actual mass / theoretical yield x 100%
= 5,781 / 6.274 x 100%
= 92.143 %

Stoichiometry calculation of theoretical yield and mass percentage yield of acetylsalicylic synthesis from the calculation above we have 6.274 g and 92.143 % respectively.

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