Calculate percent yield from actual yield and theoretical yield, with yield loss, interpretation, and a clear formula substitution for chemistry labs.
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Percent yield is a plain way to compare a real lab result with the result predicted by stoichiometry. The theoretical yield is the most product a reaction can make from the limiting reactant if the reaction goes perfectly and every bit of product is recovered. The actual yield is the product you isolate, dry, and weigh. Percent yield puts those two numbers on the same scale.
A percent yield calculator is useful because it keeps the arithmetic simple while you focus on the chemistry. If a reaction should make 10.0 g of product but you collect 8.2 g, the yield is 82.0%. That number does not say whether the experiment was good or bad by itself. It needs to be read with the reaction type, the purification method, and the amount of product handled during the lab.
In a classroom synthesis, a yield near 100% can be unusual because real experiments have losses. Some product sticks to glassware, some remains dissolved in the mother liquor, and some is lost during filtration or transfer. Organic reactions may also form side products. In analytical or industrial work, the expected range may be much tighter, but the same calculation is still used.
The percent yield formula is:
percent yield = actual yield ÷ theoretical yield × 100
Use the same mass unit for both yields. Grams are common, but milligrams or kilograms work the same way when both numbers use the same unit. The unit cancels because the calculation is a ratio. If your actual yield is in milligrams and your theoretical yield is in grams, convert one value before dividing.
The theoretical yield must be greater than zero. A zero or negative theoretical yield means the stoichiometry setup needs to be fixed. Actual yield can be zero if no product was collected. In that case, the percent yield is 0%. If the actual yield is greater than the theoretical yield, the calculator still gives the result, but the value should be reviewed before it goes into a final report.
| grams and grams | Use directly |
| mg and mg | Use directly |
| mg and g | Convert before dividing |
Suppose a student prepares copper(II) sulfate crystals. The limiting reactant calculation predicts a theoretical yield of 12.50 g. After filtering, rinsing, and drying the crystals, the student weighs 10.80 g of product. The percent yield is found by placing the actual yield over the theoretical yield.
10.80 g ÷ 12.50 g × 100 = 86.4%
The yield loss is the difference between the theoretical yield and the actual yield. In this example, 12.50 g minus 10.80 g equals 1.70 g. That lost amount does not point to one cause by itself. It simply tells you how much product was not present in the final weighed sample compared with the stoichiometric prediction.
A good lab discussion connects the number to observations. If the filtrate was still strongly colored, product may have remained in solution. If crystals were stuck to filter paper or the beaker, transfer loss is likely. If the reaction mixture was heated too briefly, the reaction may not have gone to completion. The percent yield calculation gives the value, while the lab notes help explain why the value happened.
"The actual yield was 10.80 g and the theoretical yield was 12.50 g. The percent yield was 86.4%. Some loss likely occurred during filtration and transfer because small crystals were visible on the filter paper and inside the beaker."
A percent yield above 100% means the measured actual yield is larger than the theoretical yield. The calculation is not broken. It is reporting exactly what was entered. The chemistry question is why the collected sample weighed more than a pure dry sample should weigh.
The most common reason is water or solvent left in the product. Wet crystals can look dry on the outside while still holding liquid between crystals or inside a filter cake. The balance records all of that mass. A sample can also include impurities such as unreacted starting material, salt, sand, filter paper fibers, or side products that were not removed during washing or recrystallization.
A yield over 100% can also come from math or measurement issues. The theoretical yield may have been calculated from the wrong limiting reactant, a molar mass may have been copied incorrectly, or a container may not have been tared properly. In a report, do not treat a yield above 100% as a success. Explain the likely source and describe what could be done to check it.
Percent yield should be reported with a reasonable number of significant figures. In many general chemistry labs, the answer is limited by the measured actual yield, the theoretical yield, or both. If the actual yield is 8.42 g and the theoretical yield is 10.0 g, both values have three significant figures, so 84.2% is a good reported result.
Avoid reporting a long calculator display such as 84.2000000%. Extra digits do not make the measurement more precise. They can make the report look less careful because the balance and the stoichiometric calculation did not support that many digits.
If your teacher gives a specific rule, use that rule. Some classes ask for one decimal place for percent yield even when significant figures would allow a slightly different format. In that case, the class convention is part of the assignment. The important point is to keep the raw masses, the formula substitution, and the final percentage consistent.
| Substitution | Report as |
|---|---|
| 8.42 g ÷ 10.0 g × 100 | 84.2% |
| 0.315 g ÷ 0.400 g × 100 | 78.8% |
| 14 g ÷ 20 g × 100 | 70% |
A percent yield is strongest when it is paired with a short, specific explanation. Instead of writing only that mistakes happened, name the step that could have changed the mass. Product spilled during transfer, crystals lost during washing, incomplete drying, and an unwashed precipitate are all clearer than a vague statement.
Use the interpretation band as a starting point, not as a final grade for the experiment. A 45% yield might be acceptable for a difficult multi-step synthesis, but poor for a simple precipitation reaction. An 85% yield may be excellent in a teaching lab, but still worth improving in a process chemistry setting. The expected range depends on the reaction, scale, product solubility, and purification method.
When you compare groups, make sure everyone used the same definition of actual yield. Weighing a wet filter paper with product, weighing only the dry product, or forgetting to subtract the mass of a weigh boat will give different values. Good percent yield work starts with consistent weighing and clear notes about what was placed on the balance.
Percent yield compares the amount of product you actually collected with the amount predicted by stoichiometry. The formula is actual yield divided by theoretical yield, then multiplied by 100.
Yes, the calculation can give a value above 100%, but it usually means the collected material is not pure dry product. Common causes include wet crystals, leftover solvent, impurities, balance error, or a theoretical yield that was calculated from the wrong limiting reactant.
Actual yield goes on top because it is the measured product from the lab. Theoretical yield goes on the bottom because it is the maximum product predicted from the balanced equation and limiting reactant.
Use the same unit for both values, such as grams, milligrams, or kilograms. Percent yield is a ratio, so the units cancel. Do not mix grams and milligrams unless you convert one value first.
Report percent yield with the same number of significant figures as the least precise measured or calculated yield used in the ratio. In a lab report, also keep the actual yield and theoretical yield in the same unit so the calculation is easy to check.
A low yield can come from incomplete reaction, side reactions, product lost during transfer, product left in the filtrate, poor crystallization, or not enough drying time before weighing. Check the limiting reactant math first, then review the physical steps used to collect and purify the product.