Theoretical Yield Definition in Chemistry

Theoretical yield is the quantity of a product obtained from the complete conversion of the limiting reactant in a chemical reaction. It is the amount of product resulting from a perfect (theoretical) chemical reaction, and thus not the same as the amount you'll actually get from a reaction in the lab. Theoretical yield is commonly expressed in terms of grams or moles.

In contrast to theoretical yield, the actual yield is the amount of product actually produced by a reaction. Actual yield is usually a smaller quantity because few chemical reactions proceed with 100% efficiency because of loss recovering the product and because other reactions may be occurring that reduce the product. Sometimes an actual yield is more than a theoretical yield, possibly because of a secondary reaction that yields additional product or because the recovered product contains impurities.

The ratio between actual yield and theoretical yield is most often given as percent yield:

Percent yield = Mass of actual yield / Mass of theoretical yield x 100 percent

How to Calculate Theoretical Yield

Theoretical yield is found by identifying the limiting reactant of a balanced chemical equation. In order to find it, the first step is to balance the equation, if it's unbalanced.

The next step is to identify the limiting reactant. This is based on the mole ratio between the reactants. The limiting reactant is not found in excess, so the reaction cannot proceed once it is used up.

To find the limiting reactant:

1. If the quantity of reactants is given in moles, convert the values to grams.
2. Divide the mass of the reactant in grams by its molecular weight in grams per mole.
3. Alternatively, for a liquid solution, you can multiply the amount of a reactant solution in milliliters by its density in grams per milliliter. Then, divide the resulting value by the reactant's molar mass.
4. Multiply the mass obtained using either method by the number of moles of reactant in the balanced equation.
5. Now you know the moles of each reactant. Compare this to the molar ratio of the reactants to decide which is available in excess and which will get used up first (the limiting reactant).

Once you identify the limiting reactant, multiply the moles of limiting reaction times the ratio between moles of limiting reactant and product from the balanced equation. This gives you the number of moles of each product.

To get the grams of product, multiply the moles of each product by its molecular weight.

For example, in an experiment in which you prepare acetylsalicylic acid (aspirin) from salicylic acid, you know from the balanced equation for aspirin synthesis that the mole ratio between the limiting reactant (salicylic acid) and the product (acetylsalicylic acid) is 1:1.

If you have 0.00153 moles of salicylic acid, the theoretical yield is:

Theoretical yield = 0.00153 mol salicylic acid x (1 mol acetylsalicylic acid / 1 mol salicylic acid) x (180.2 g acetylsalicylic acid / 1 mole acetylsalicylic acid

Theoretical yield = 0.276 grams acetylsalicylic acid

Of course, when preparing aspirin, you'll never get that amount. If you get too much, you probably have excess solvent or else your product is impure. More likely, you'll get much less because the reaction won't proceed 100 percent and you'll lose some product trying to recover it (usually on a filter).