[BRIT]

We did an experiment on synthesizing aspirin...
We had 2.188g of salicylic acid (C7H6O3)=MW 138.1226
      and 4.139g of acetic anhydride (C4H6O3)=MW 102.0896

equation:
C7H6O3+C4H6O3-->C9H8O4+CH3COOH

I got as far as...

Salicylic Acid:
2.188g x 1 mol = 0.0158 mol
           138.1226

Acetic Anhydride:
4.139g x 1 mol =0.0405 mol
           102.0896

Acetic Anhydride is the limiting reageant...now I'm lost on how to find the theoretical yield...we have to find it in grams & moles...

[Yggdrasil]

Why do you think acetic anhydride is the limiting reagent?  There are more moles of acetic anhydride than moles of salicylic acid.

Anyway, to figure out the theoretical yield, you use the stoichiometric coefficients from your balanced chemical reaction.  From the balanced chemical reaction, you know that one mole of salicylic acid reacts to produce one mole of aspirin.  So, if you react 0.0158 moles of salicylic acid, how many moles of aspirin would you expect to produce?  What mass of aspirin would this amount of moles equate to?

[BRIT]

Oops. I knew salicylic acid was the LR.

Okay, next to the question it says (GMM: 180 g mol-1)

so would it be...

.0158 x 1 mol = 8.78 mol x 10 to 5
           180 g

how do you find grams then?
I'm sorry, I'm just really confused. That seems like a weird answer. I'm really behind in chemistry, I know!

[Yggdrasil]

To get from grams to moles, you divide by the molar mass.

To get from moles to grams, you multiply by the molar mass.

[BRIT]

.0158 x 1 mol = 8.78 mol x 10 to 5
           180 g

So that part is actually right?
& for grams i do....

.0158g x 180g= 2.844 g
              1 mol

Is this right? & which one do i use for percient yield, grams or moles? THANKSSSS!

[Yggdrasil]

.0158 x 1 mol = 8.78 mol x 10 to 5
           180 g

I don't understand what you are trying to do here?

But, since you are reacting 0.0158 mol of salicylic acid and 1 mol of salicylic acid produces 1 mol of aspirin, your theoretical yield will be 0.0158 mol of aspirin.  Converting this to mass using the molecular weight of aspirin gives 2.844g of aspirin.  You can use either value as your theoretical yield when calculating percent yield.