How to Calculate Percent Yield Step by Step
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Calculating percent yield starts with two quantities: the amount of product you actually obtained and the amount you theoretically could have obtained. Comparing them shows how closely an experiment approached its predicted maximum.
The formula
The percent yield formula is actual yield divided by theoretical yield, multiplied by 100. Suppose a reaction should theoretically produce 20 grams of product, but the experiment produces 16 grams. Divide 16 by 20 to get 0.8, then multiply by 100. The percent yield is 80 percent.
This is the basic method for calculating percent yield, but the two yields must refer to the same product and use compatible units. Do not divide grams by moles or compare the yield of one substance with the predicted amount of another. Convert quantities first if the question gives them in different forms.
Find theoretical yield before using percent yield
In many chemistry questions, theoretical yield is not given directly. You may need to calculate it from the reactants. Start with a balanced chemical equation, convert the available reactant amount to moles, identify the limiting reactant when necessary, and use the equation's mole ratio to predict the number of moles of product.
You can then convert the predicted product into the requested unit, often grams. Only after that should you calculate the percent yield. Keeping the stoichiometry and percent calculation as separate stages makes the problem easier to check. If the theoretical yield is wrong, the final percentage will also be wrong even if the division is performed perfectly.
Why actual yield is often lower
Real experiments rarely collect every particle of the product predicted by a simple equation. A reaction may not go to completion, competing reactions may occur, or some product may remain in equipment. Material can also be lost during filtering, transferring, drying, or purification. These practical losses often make actual yield smaller than theoretical yield.
When calculating percent yield, a result above 100 percent should make you investigate rather than immediately assume chemistry has broken its own rules. The measured product may contain water, solvent, impurities, or another substance that increases its mass. Measurement errors can also matter. A percent yield calculation is therefore both a mathematical result and a clue about the experiment.
A full worked example
Suppose you burn 12 grams of magnesium in excess oxygen to make magnesium oxide and collect 18 grams of product. First find the theoretical yield. The equation is 2Mg + O2 gives 2MgO, and magnesium's molar mass is about 24 grams per mole, so 12 grams is 0.5 moles. The ratio of Mg to MgO is one to one, so you expect 0.5 moles of MgO. Its molar mass is about 40 grams per mole, so the theoretical yield is 20 grams. Now the percent yield: 18 divided by 20 is 0.9, so 90 percent.
Notice that the chemistry took four steps and the percentage took one. If a question gives 24 grams of product, stop before dividing. A yield of 120 percent means some magnesium reacted with nitrogen in the air, or the product was weighed wet, or the mass of the crucible crept into the reading, and saying which is the mark-winning part of the answer.
The takeaway
Calculating percent yield is straightforward once both yields are known: divide actual yield by theoretical yield and multiply by 100. The harder part is often finding a correct theoretical yield through stoichiometry. Keep units consistent, identify the limiting reactant when needed, and treat unusual percentages as reasons to check your work.