Percent Yield vs Percent Error in Chemistry

What Percent Yield Measures

Percent yield compares the actual amount of product recovered to the theoretical amount predicted by the balanced equations. It is a measure of efficiency, not accuracy. A reaction can have a beautiful 98 percent yield and still be badly wrong, because the two questions are different.

What Percent Error Measures

Percent error compares an experimental value to an accepted or theoretical value. It is a measure of accuracy. It is the right figure for physical constants such as melting point, boiling point or a measured density, where a true value is independently known.

Why Yield Alone Can Hide a Bad Experiment

Recovering almost all of the expected product does not prove the product was pure or that the reaction was the one you thought. An impure sample weighing slightly more than expected can show a yield over 100 percent. A percent yield figure is meaningful only alongside evidence that the recovered material was actually the intended product.

Calculating Percent Yield

Percent yield equals actual yield divided by theoretical yield, multiplied by 100. Work out the theoretical yield from the limiting reagent first, then divide. If the theory predicts 12.4 grams and you recovered 11.2 grams, the yield is 11.2 / 12.4 x 100, which is about 90.3 percent.

Calculating Percent Error for a Constant

If the accepted melting point of a compound is 122 degrees Celsius and your measured value was 119, the absolute error is 3 degrees, the relative error is 3 / 122, and the percent error is about 2.46 percent. Use the accepted value as the denominator every time, never your own reading.

A Yield Over 100 Percent Means a Problem

A yield above 100 percent is not good news, it is a warning. Usual causes include wet product that was not fully dried, solvent still present in the weighing, impurities adding mass, or an error in the theoretical calculation. Report it honestly and investigate rather than quietly reporting the number.

Which One Your Report Needs

Synthesis experiments usually need percent yield. Characterisation experiments measuring a physical constant need percent error. Reports that do both, such as synthesising a product and then measuring its melting point, need both, and the two figures answer completely different questions about how the experiment went.

Reporting Both Together

A high percent yield with a large percent error in melting point suggests the reaction worked but the product is impure. A good melting point with a low yield suggests the reaction was inefficient but produced clean material. Reading the pair together is far more informative than either figure on its own.