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Posted by on Tuesday, November 27, 2012 at 1:34am.

I asked this already but for some reason the answer I got isnt being accepted online
Question:
In the laboratory, 3.50 grams of salicylic acid were reacted with 7.20 ml of acetic anhydride (density equal to 1.08 g/ml). Determine the limiting reagent.

I did:
salicylic acid + acetic anhydride → acetyl salicylic acid + acetic acid
C6H4(OH)COOH + (CH3CO)2O → CH3COOC6H4COOH + CH3COOH
moles C6H4(OH)COOH = (mass C6H4(OH)COOH) / (mole mass C6H4(OH)COOH)
moles C6H4(OH)COOH = (3.50 g) / (138.12354 g/mol)
moles C6H4(OH)COOH = 0.025396 mol
density (CH3CO)2O = [mass (CH3CO)2O] / [volume (CH3CO)2O]
or
mass (CH3CO)2O = [density (CH3CO)2O][volume (CH3CO)2O]
mass (CH3CO)2O = [1.08 g/mL][7.20 mL]
mass (CH3CO)2O = 7.776 g
moles (CH3CO)2O = [mass (CH3CO)2O] / [mole mass (CH3CO)2O]
moles (CH3CO)2O = [7.776 g] / [102.09024 g/mL]
moles (CH3CO)2O = 0.0761679 mol

So the answer would be salycic acid b/c its the lower number 0.025 mol

BUT my answer is incorrect apparently. Is here something I did wrong?

  • CHEM HELP - , Tuesday, November 27, 2012 at 11:37pm

    Thanks for showing your work.
    Nope. You're right on target and your numbers are correct. Salicylic acid is the limiting reagent. One of several things may be the problem.
    1. You posted the wrong numbers.
    2. You didn't post all of the problem.
    3. The reaction you have is wrong.
    4. The problem was given to you with the wrong numbers.
    5. The database (key) is wrong.

    But I can assure you that if the numbers you posted are correct and the equation is correct, then the limiting reagent is salicylic acid. In MOST of the aspirin problems the acetic anhydride is the limiting reagent but that isn't true here.

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