Enter the actual yield you collected and the theoretical yield from your equation, and the page gives the percent yield with every step. A second mode works the other way and shows how much product you should expect at a given yield. UK courses call this percentage yield.
Pick a question and enter the numbers
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Your answer
Your answer shows up here once you fill in both numbers.
What to enter
- Actual yield
- The mass or amount of product you really collected and weighed in the lab.
- Theoretical yield
- The most product the reaction could make, worked out from the balanced equation and the limiting reagent. This page does not work it out for you, but the example below shows how.
- Yield to expect
- Enter the theoretical yield and the percent yield you expect. The page shows the amount you would collect.
Worked examples
From the equation to the percent yield
Hydrogen burns in oxygen: 2 H₂ + O₂ → 2 H₂O. Start with 4.00 g of hydrogen and plenty of oxygen. The molar mass of H₂ is 2.016 g/mol, so you have 1.984 mol. The equation makes 1 mol of water for every 1 mol of hydrogen, and water has a molar mass of 18.015 g/mol, so the theoretical yield is 35.74 g. You collect 30.2 g. What is the percent yield?
- Divide the actual yield by the theoretical yield: 30.2 ÷ 35.74 = 0.844992.
- Multiply by 100: 0.844992 × 100 = 84.499161%.
The percent yield is 84.499161%. The rest was lost along the way, for example as vapour, in transfers, or in a side reaction.
How much to expect
The theoretical yield of a product is 12.5 g. A technique that usually gives 80% yield is used. How much will you collect?
- Turn the percent yield into a decimal: 80 ÷ 100 = 0.8.
- Multiply by the theoretical yield: 12.5 × 0.8 = 10.
You should collect about 10 g.
A yield over 100%
You weigh 11 g of product and the theoretical yield is 10 g.
- Divide the actual yield by the theoretical yield: 11 ÷ 10 = 1.1.
- Multiply by 100: 1.1 × 100 = 110%.
That gives 110%. A yield above 100% cannot be real, so it means something is off, often a product that is still wet, or has leftover reagent or solvent in it.
Finding the theoretical yield
The theoretical yield is the mass of product you would get if every bit of the limiting reagent turned into product. Work it out in four steps.
- Balance the equation.
- Turn the mass of each reactant into moles by dividing by its molar mass.
- Find the limiting reagent, which is the one that runs out first, using the mole ratio in the equation.
- Use the mole ratio to turn the moles of limiting reagent into moles of product, then multiply by the molar mass of the product.
Once you have that number, enter it as the theoretical yield above.
Why a yield below 100% is normal
Reactions rarely go all the way, and some product is lost in every step of the lab. Some of it stays on filter paper, some stays in the flask, and some is made in side reactions you did not plan for. Your course will tell you what range counts as a good yield for the experiment.
How it is calculated
- Divide the actual yield by the theoretical yield.
- Multiply by 100 to turn it into a percent.
- To go the other way, turn the percent yield into a decimal and multiply it by the theoretical yield.
Percent yield = actual yield ÷ theoretical yield × 100
Expected actual yield = theoretical yield × percent yield ÷ 100
Assumptions to keep in mind
- The actual and theoretical yield must use the same unit, such as grams or moles.
- You supply the theoretical yield. The page does not balance equations or find the limiting reagent.
- If the theoretical yield is 0, the page asks you to check it, because you cannot divide by 0.
- The page does the arithmetic only. Check the significant figures your course expects.
Mistakes that give the wrong answer
Using the wrong reagent for the theoretical yield
The theoretical yield comes from the limiting reagent, not from the reagent you have the most of.
Swapping actual and theoretical
The actual yield goes on top. If you get a number that is far above 100%, check the order.
Mixing grams and moles
Use one unit for both numbers. Convert before you divide.
Reporting more than 100% without comment
A result over 100% means something is wrong with the product or the measurement. Say so in your report and name the likely cause.
Questions students ask
How do I calculate percent yield?
Divide the actual yield by the theoretical yield, then multiply by 100. For 30.2 g collected against a theoretical 35.74 g, that is 30.2 ÷ 35.74 × 100 = 84.5%.
What is theoretical yield?
It is the most product a reaction could make, based on the balanced equation and the amount of the limiting reagent. You work it out before the experiment.
Can percent yield be over 100%?
Not in a real reaction. If you get more than 100%, the product probably has water, solvent or leftover reagent in it, or there was a measuring mistake.
What is the difference between percent yield and percent error?
Percent yield compares how much product you made with the most you could make. Percent error compares a measured value with an accepted value. The percent error calculator covers the second one.
Is percent yield the same as percentage yield?
Yes. Percentage yield is the wording used in many UK courses. The calculation is the same.
