# chemistry - synthesis of aspirin

posted by on .

This is an experiment for a synthesis of aspirin lab

mass of salicylic acid = 2.005 g

mass of acetic acid = ?

moles of acetic acid = ?

volume of acetic anhydride = 4 ml

mass of acetic anhydride (Use 1.08 g/ml for the liquid density) = (1.08 g/ml)
(4 ml) = 4.32 g

moles of acetic anhydride (C4H6O3; molar mass = 102 g) = 4.32 g / 102 g = .042 mol

mass of filter paper = .196 g

mass of filter paper plus aspirin = 2.386 g

mass of weighing tray = 1.900 g

mass of filter paper, aspirin, and weighing tray = 4.286 g

mass of aspirin = 2.19 g

moles of aspirin (C9H8O4; molar mass = 180 g) = 2.19 g / 180 g = .012 mol

I understand now how to calculate the theoretical, actual and percent yield of aspirin. But I don't know how to calculate the mass of the acetic acid.

thank you for any help

• chemistry - synthesis of aspirin - ,

I checked your values for mols aspirin and it looks good except I would carry it out one more place. You have 3 significant figures in all of the other numbers (if the 4 mL is 4.00 mL); therefore, you can have 3 in the 0.012 number. Same for mols acetic anhydride; again I'm assuming the 4 mL is 4.00 mL.
Did you have any water? Are you supposed to assume there was no water present in the calculation of the mass acetic acid?
If you had no H2O present then salicylic acid is the limiting reagent and it will produce (theoretically) that many mols of acetic acid.
sal acid + acetic anhydride =>aspirin + acetic acid and everything is 1:1:1:1.
If aspirin was about 80% yield, the acetic acid should be about 80% yield also.

• chemistry - synthesis of aspirin - ,

ok therefore, the mass of the aspirin is equal to the mass of the acetic acid since the ratio is 1:1. Right?

• chemistry - synthesis of aspirin - ,

wait, i misunderstand. You stated that if there's no H2O present then salicylic acid is the limiting reagent. Therefore salicylic acid number of moles is the same as the number of moles of acetic acid???? If this is so, then the mass of acetic acid would also be 2.005 g, right?

• chemistry - synthesis of aspirin - ,

No. Mass of acetic acid = mols acetic acid x molar mass acetic acid and that is the theoretical yield of acetic acid in grams.
I don't know the exact instructions you have but if I were asked to calculate grams acetic acid produced I would do as above then multiply by the percent yield for the aspirin to arrive at g acetic acid I would expect to be produced. My assumption is that percent yield for aspirin and percent yield for acetic acid is the same.