salicylic acid + acetic anhydride ==> aspirin + acetic acid
To find theoretical yield of aspirin, we first find moles of salicylic acid used, and assume acetic anhydride in present in excess so that it isn't limiting. We then find theoretical moles of aspirin, and finally convert that to theoretical mass.
moles salicylic acid used = 1.5000 g x 1 mol/138.121 g = 0.0114398 moles
moles aspirin formed = 0.0114398 moles aspirin since it is a 1:1 mole ratio in the balanced equation
mass aspirin = theoretical yield = 0.0114398 moles x 180.158 g/mol = 2.0610 g