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Organic Reaction Calculator
Enter your reactant masses, molar masses, and equation coefficients to find the limiting reagent, theoretical yield, percent yield, and atom economy of an organic reaction.
What an organic reaction calculator can and cannot do
Every organic synthesis starts with the same question: how much product could this reaction give, and how close did I get? This calculator answers it. It takes the amounts of your starting materials, converts them to moles, finds the reagent that will run out first, and uses that reagent to calculate the maximum mass of product. Then it compares that maximum with what you actually weighed at the end of the experiment.
It is a stoichiometry tool, so you need to know the balanced equation before you start. It does not predict the products of an unfamiliar reaction, suggest reagents, or draw mechanisms. Those jobs need a reaction database or a textbook. What it does well is the arithmetic that students and bench chemists repeat after nearly every experiment, without the rounding slips that creep into a hand calculation.
The formulas behind the results
Moles = Mass ÷ Molar Mass
Every calculation starts here. 2.00 g of salicylic acid (138.12 g/mol) is 2.00 ÷ 138.12 = 0.01448 mol, or 14.48 mmol.
Limiting reagent
Smallest value of (moles ÷ coefficient)
Dividing by the coefficient puts every reactant on the same footing, so the smallest result is the reagent that limits the reaction.
Theoretical yield
(Limiting moles ÷ its coefficient) × product coefficient × product molar mass
For the aspirin example: 0.01448 × 1 × 180.16 = 2.61 g of aspirin at most.
Percent yield
(Actual yield ÷ Theoretical yield) × 100
1.80 g isolated from a 2.61 g theoretical yield gives a percent yield of 69%.
Atom economy
Product coefficient × molar mass ÷ Σ(reactant coefficient × molar mass) × 100
180.16 ÷ (138.12 + 102.09) gives 75.0% for aspirin, because the rest of the reactant mass leaves as acetic acid.
How to use it, step by step
Step 1: Balance the equation. The coefficients in front of each compound tell the calculator the mole ratio. If a reaction is 1 to 1, enter 1 for everything. If two moles of ethanol react with one mole of adipic acid, enter 2 for ethanol and 1 for the acid.
Step 2: Enter mass and molar mass for each reactant. Use the mass you actually weighed. For a liquid, multiply volume by density first. Molar mass comes from the formula, or from the label on the reagent bottle.
Step 3: Enter the product. You only need its molar mass and its coefficient. If there are several products, pick the one you are isolating.
Step 4: Add your actual yield. Weigh the dry, purified product and enter it in the same unit as the reactants. Leave it blank to see the theoretical yield alone.
Step 5: Read the results. The reagent table lists each reactant in millimoles and as equivalents relative to the limiting reagent. An equivalents value of 1.00 marks the limiting reagent, and a value of 3.65 means you used 3.65 times the amount needed.
Molar masses of common organic lab reagents
These values are rounded to two decimals. Always check the number printed on your own reagent bottle, because hydrates and salts have different molar masses from the anhydrous compound.
| Compound | Formula | Molar Mass (g/mol) |
|---|---|---|
| Methanol | CH₃OH | 32.04 |
| Ethanol | C₂H₅OH | 46.07 |
| Acetic acid | C₂H₄O₂ | 60.05 |
| Acetic anhydride | C₄H₆O₃ | 102.09 |
| Ethyl acetate | C₄H₈O₂ | 88.11 |
| Benzaldehyde | C₇H₆O | 106.12 |
| Benzoic acid | C₇H₆O₂ | 122.12 |
| Salicylic acid | C₇H₆O₃ | 138.12 |
| Aspirin | C₉H₈O₄ | 180.16 |
| Bromobenzene | C₆H₅Br | 157.01 |
Worked examples
Aspirin: 2.00 g salicylic acid + 5.40 g acetic anhydride
Moles: 0.01448 and 0.05289. Both coefficients are 1, so salicylic acid is limiting. Theoretical yield: 2.61 g. With 1.80 g isolated, the percent yield is 69.0%.
Fischer esterification: 6.01 g acetic acid + 9.21 g ethanol
Moles: 0.1001 and 0.1999. Acetic acid is limiting and ethanol is used in about 2 equivalents. Theoretical yield of ethyl acetate: 8.82 g. With 6.20 g isolated, the yield is 70.3%.
Diethyl adipate: 14.61 g adipic acid + 18.40 g ethanol
The acid gives 0.09997 mol and the ethanol gives 0.3994 mol, which is 0.1997 after dividing by its coefficient of 2. Adipic acid is limiting. Theoretical yield: 20.22 g. With 17.5 g isolated, the yield is 86.5%.
Three-step synthesis: 85%, 70%, and 90%
Multiply the step yields as decimals: 0.85 × 0.70 × 0.90 = 0.5355. The overall yield is about 54%, which shows why long syntheses need very efficient individual steps.
Why real yields fall short of the theoretical yield
Reversible reactions such as esterification stop before all of the limiting reagent is used. Using an excess of one reagent or removing water pushes the yield higher.
Some starting material forms byproducts instead of your target. Temperature, reaction time, and reagent purity all change how much.
Product stays behind in the separatory funnel, on filter paper, in the mother liquor, and on glassware walls. Every transfer costs a little.
Leftover solvent inflates the mass and can push a calculated yield above 100%. Dry the product to constant mass before you weigh it.
More CalcMora tools for lab and study math
Chemistry calculations lean on several everyday math skills, and a few CalcMora tools cover them. When you report a yield, the right number of digits matters, and the significant figures calculator checks your rounding. Comparing a measured value with an expected one is a percentage problem, so the percentage calculator handles percent change and percent difference. Concentrations and tiny masses often use scientific notation, and the exponent calculator works with powers of ten. For long chains of arithmetic on an exam, the 991ES calculator online gives you a scientific calculator in the browser. If a supplier lists chemicals in pounds, the kg to lbs calculator converts them to a metric mass. And once your lab reports are graded, the final grade calculator shows what score you need on the exam.
Organic reaction calculator — FAQ
What does an organic reaction calculator do?
It does the stoichiometry for a reaction you already know. You enter the mass, molar mass, and balanced-equation coefficient for each reactant, plus the molar mass and coefficient of the product you want. The calculator converts masses to moles, finds the limiting reagent, works out the theoretical yield, and compares it with your actual yield to give a percent yield. It does not predict what products a reaction will form, and it does not draw mechanisms.
How do I find the limiting reagent in an organic reaction?
Convert each reactant to moles by dividing its mass by its molar mass, then divide each mole value by its coefficient from the balanced equation. The reactant with the smallest result is the limiting reagent, because it runs out first and caps how much product can form. In the aspirin example, 2.00 g of salicylic acid is 0.01448 mol and 5.40 g of acetic anhydride is 0.05289 mol. Both have a coefficient of 1, so salicylic acid is limiting.
How do I calculate percent yield?
Percent yield = (actual yield ÷ theoretical yield) × 100. The actual yield is the mass of purified product you weighed, and the theoretical yield is the maximum mass the limiting reagent could produce. If the theoretical yield is 2.61 g and you isolated 1.80 g, the percent yield is about 69%. Always use the same mass unit for both numbers.
Why is the percent yield of an organic reaction almost always below 100%?
Several things reduce yield in real organic chemistry. Many reactions are reversible and stop at an equilibrium mixture. Side reactions turn some starting material into unwanted products. Product is also lost during extraction, filtration, recrystallization, and transfers between flasks. Each purification step gives up a little material, which is why multi-step syntheses have lower overall yields than any single step.
What is a good percent yield?
It depends on the reaction. As a rough guide, a single step above 90% is excellent, 70% to 90% is good, and 40% to 70% is common in teaching labs and difficult transformations. Yields matter more in long syntheses, because the loss compounds: three steps at 85%, 70%, and 90% leave an overall yield of about 54%.
What does it mean if my percent yield is over 100%?
A yield above 100% means the product mass includes something besides pure product. The most common causes are leftover solvent or water, an incompletely dried solid, and impurities such as unreacted starting material. It can also come from an entry error, such as the wrong molar mass or coefficient. Dry the product further, check purity by melting point or NMR, and re-check the numbers you entered.
What is atom economy and why is it shown?
Atom economy measures how much of the mass of the reactants ends up in the desired product on paper: (coefficient × molar mass of the product) ÷ (sum of coefficient × molar mass for all reactants) × 100. It is a green chemistry measure. A reaction can have a high percent yield and still waste a lot of atoms as byproducts, so atom economy shows that side of the process. The calculator only counts the reactants you enter, so add every reactant for a meaningful value.
How do I enter a liquid reagent measured in mL?
Convert the volume to mass first: mass = volume × density. For example, 5.00 mL of a liquid with a density of 1.08 g/mL has a mass of 5.40 g. Enter that mass in the calculator. Check the density on the reagent bottle or the supplier's safety data sheet, because it changes with temperature and purity.
This tool is for educational purposes only. Always verify important results with a qualified professional.
