Stoichem stuff problems

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Aspirin (C9H8O4) is produced from salicylic acid (C7H6O3), and acetic anhydride (C4H6O3), as shown below.

C7H6O3 + C4H6O3 → C9H8O4 + HC2H3O2

(a) How much salicylic acid is required to produce 1.0  102 kg of aspirin, assuming that all of the salicylic acid is converted to aspirin?

(b) How much salicylic acid would be required if only 56% of the salicylic acid is converted to aspirin?

(c) What is the theoretical yield of aspirin if 181 kg of salicylic acid is allowed to react with 124 kg of acetic anhydride?


Oct 22nd, 2017

Thank you for the opportunity to help you with your question!

Your structure for acetic acid is correct and so is your balanced equation. 

To find theoretical yield, you will first need to find the limiting reactant. First convert everything to moles. 

Salicylic acid (138 g/mol): 
185 kg = 185000g 
185000 g / (138 g/mol) = 1341 moles (wow, large scale!) 

Acetic Anhydride (102 g/mol): 
125kg = 125000g 
125000g / (102 g/mol) = 1225 moles 

Based on your balanced equation, there is a one to one mole ration between salicylic acid and acetic anhydride, which makes things easier. Basically, you need to determine which reactant runs out first (limiting reactant). Because the mole ratio is 1:1, in this case the limiting reagent is the one with the least number of moles (acetic anhydride). 

Because there is a 1:1 mole ratio between acetic anhydride and aspirin (every 1 mole of acetic anhydride that you use up produces one mole of aspirin), your theoretical yield of aspirin (in moles) is 1225 moles. Theoretical yields are normally reported in grams, so you'll need to convert 1225 moles of aspirin to gram of aspirin (144 g/mol). 

I get 220500g. Based on significant figures, it would be 221kg. That's a lot of aspirin.

Please let me know if you nea case for torture by michale levined any clarification. I'm always happy to answer your questions.
Jul 24th, 2015

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