Aspirin (C9H8O4) is synthesized by the reaction of salicylic acid (C7H6O3) with acetic anhydride, C4H6O3.

Question

Aspirin (C9H8O4) is synthesized by the reaction of salicylic acid (C7H6O3) with acetic anhydride, C4H6O3. 2 C7H6O3 + C4H6O3 −→ 2 C9H8O4 + H2O When 17 g of C7H6O3 and 17 g of C4H6O3 react, which is the limiting reagent? Answer choices 1. C7H6O3 2. C4H6O3

How much of the excess reactant is used when the reaction is complete?

Answer choices 1. 0.0615362 2. 0.108593 3. 0.0868747 4. 0.097734 5. 0.0579165 6. 0.0542967 7. 0.0941142 8. 0.0361978 9. 0.0398176 10. 0.104974 Answer in units of mol.

And What mass of aspirin is formed?

Answer choices 1. 23.4822 2. 33.9188 3. 22.1777 4. 13.0457 5. 16.9594 6. 24.7868 7. 27.3959 8. 36.5279 9. 19.5685 10. 18.264 Answer in units of g.

Answer 1

Hi, Reel,

I've actually DONE this reaction (years ago) - if you should ever plan to do this, be sure you're working in a lab with a good fume hood ... needless to say, when I did this for a high school chemistry lab, the ventilation was less than ideal!!

No matter, we start with restating the chemical equation, verifying that it's a balanced equation:

2 C₇H₆O₃ + C₄H₆O₃ → 2 C₉H₈O₄ + H₂O

18 carbons on both sides of the equation, 18 hydrogens, and 9 oxygen atoms, so everything checks.

Next, we need to convert the gram amounts for each reagent (17 grams) into moles to determine the limiting reagent. To do this, we need the molecular weights of each reagent, so:

Salicylic acid: C₇H₆O₃ = (7x12.1) + (6x1.01) + (3x16.1) = 138.12 gm/mol

Acetic anhydride: C₄H₆O₃ = (4x12.1) + (6x1.01) + (3x16.1) = 102.09 gm/mol

We can now solve for the number of moles available for this reaction as follows:

Moles (salicylic acid, SA) = 17.0 gm/138.12 gm/mol = 0.123 moles

Moles (acetic anhydride, AA) = 17.0 gm/102.09 gm/mol = 0.167 moles

Based on the stoichiometry of the reaction, you react 2 molecules (or moles) of salicylic acid with one molecule (mole) of acetic anhydride. Therefore, 0.123 moles of SA would only consume 0.0615 moles of acetic anhydride. As there is considerably more AA left over, the answer for the first part of this problem is 1. C₇H₆O₃ is the limiting reagent.

The second part of the question is: "How much of the excess reactant is used when the reaction is complete?" Again, if you completely consume the limiting reagent (salicylic acid), you would use 0.0615 moles of acetic anhydride C₄H₆O₃ . Although there are way too many digits provided for all of those possible answers (strictly speaking, there should only be two significant figures in the solution as "17" grams has only two significant figures!), the most correct answer is 1. 0.0615362 moles.

Finally, "And What mass of aspirin is formed?" For this answer, we first recognize that 2 molecules of salicylic acid will generate 2 molecules of aspirin (better known as acetylsalicylic acid, by the way!). Since the stoichiometry is identical, then the number of moles of salicylic acid used/consumed will equal the number of moles of aspirin generated. In this case, we know that 0.123 moles of salicylic acid was used, so this would have generated 0.123 moles of aspirin. To calculate the answer in grams, we multiple the number of moles by the molecular weight of aspirin (C₉H₈O₄):

Molecular weight of aspirin = (9x12.1) + (8x1.01) + (4x16.1) = 180.16 gm/mol

0.123 moles x 180.16 gm/mole = 22.18 g of aspirin formed.

Once again, your available answer choices have way too many significant figures, but the best one to choose would be answer 3: 22.1777 g.

'Hope that helps, Reel,

Chris