How to Find Theoretical Yield and Percent Yield
Work out theoretical yield from the limiting reactant, then turn a measured mass into a percent yield and read a result above 100 percent.
What percent yield actually measures
Percent yield compares the amount of product you recovered against the most the reaction could ever make. The actual yield is what you weigh out after the reaction, purification, and drying are done. The theoretical yield is the ceiling set by stoichiometry, assuming every limiting-reactant molecule converts and nothing is lost. Dividing actual by theoretical and multiplying by 100 turns that comparison into a single efficiency figure, which is why a chemist quoting a 92% yield is telling you the route is both clean and well behaved.
Finding the theoretical yield from the limiting reactant
Start by balancing the equation so the mole ratios are correct. Convert the mass of each reactant to moles using its molar mass, then divide by its coefficient to see which reactant runs out first; that one is the limiting reactant. Use the mole ratio between the limiting reactant and your product to find the maximum moles of product, then multiply by the product's molar mass to return to grams. That gram figure is the theoretical yield you type into the calculator alongside your measured actual yield.
Why real yields fall short of 100 percent
Almost no preparation reaches its theoretical maximum. Some reactant is consumed by side reactions that make byproducts instead of your target. Reversible reactions settle at equilibrium before every molecule converts. Even a reaction that goes to completion loses product to mechanical transfer, incomplete precipitation, adsorption onto glassware, and losses during recrystallization or chromatography. Reporting the percent yield honestly, alongside those losses, is part of a good lab writeup rather than a sign of a failed experiment.
Reading a result above 100 percent
If the calculator returns a value over 100% it shows a warning, because you cannot make more product than stoichiometry allows. The usual culprit is mass that is not really product: trapped solvent, water from an incompletely dried solid, or an impurity co-isolated with the target. The fix is to dry the sample to constant mass, then reweigh, or to purify further and check identity by melting point or spectroscopy. A yield that drops back under 100% after drying confirms the excess was solvent all along.