If you use 1 gram of salacylic acid and 3ml of acetic anhydride & 3 drops of phosphoric acid and 1 ml of water after boiling = 0.55 g of aspirin. What is the % yield?

Answer 1

You must first determine which is the limiting reagent; i.e., salicylic acid or acetic anhydride. And you need the density of acetic anhydride to do that.
mols salicylic acid = grams/molar mass = ?
mols acetic anhydride = grams/molar mass. grams you get by grams = volume x density.

Convert mols S.A. to mols of the product.
Do the same for acetic anhydride.
It is likely you will obtain two different answers; the correct one in limiting reagent problems is ALWAYS the smaller one and the reagent producing that number is the limiting reagent.

mols product x molar mass = grams. This is the theoretical yield (T. E.)

%yield = (actual yield/T. E.)*100 = ?