The process we will use is simple. First, balance the chemical equation. In this case, we have the following equation:
C7H6O3 (Salicylic Acid) + C4H6O3 (Acetic Anhydride) => C9H8O4 (Aspirin) + C2H4O2 (Acetic Acid)
This is already balanced, and the reactants have a one to one ratio with the product. Then, we calculate the molar mass of each reactant and find the limiting reactant. The limiting reactant defines the maximum theoretical product yield. The molar masses are 138.12 g/mol for salicyclic acid and 102.09 g/mol for acetic anhydride. To find out how many moles we have of each reactant, divide the mass of the reacted chemical by the molar mass. For example, we have 1.5 g / 138.12 g/mol = .0108 mol of salicyclic acid. Likewise, we have .0196 mol of acetic anhydride. In this case, salicyclic acid is the limiting agent, and the theoretical maximum molar yield of aspirin is .0108 mol.
To calculate percent yield, divide the experimental yield by the theoretical maximum. Here, the reaction yielded .00832 mol of aspirin, and the theoretical maximum is .0108 mol. The percent yield is thus 76.66%.