How this instrument works
Theoretical yield is the mass of product a reaction would produce if every mole of the limiting reagent converted completely and cleanly into product, with no side reactions, no incomplete conversion, and no loss during isolation. It is a ceiling set entirely by the balanced chemical equation and the amount of limiting reagent available — not a prediction of what will actually happen on the bench, which is almost always somewhat less.
The calculation chains three quantities together. First, moles of the limiting reagent — the reactant that runs out first and therefore caps how much product can form. Second, the stoichiometric ratio between product and limiting reagent, read directly from the balanced equation's coefficients (a 2:1 product-to-reagent ratio means each mole of reagent yields two moles of product). Third, the product's molar mass, which converts that mole quantity into a mass in grams that can actually be measured on a balance.
Theoretical yield only answers 'what's the ceiling' — it says nothing about how close a real reaction will actually get to that ceiling. That comparison is the job of this site's percent yield calculator, which takes a measured actual yield together with the theoretical figure computed here and reports the efficiency percentage between them.
- Enter the moles of your limiting reagent — the reactant that runs out first — into Limiting reagent (mol).
- Enter the mole ratio of product to limiting reagent from your balanced equation into Stoichiometric ratio (product : limiting reagent).
- Enter the product's molar mass in grams per mole into Product molar mass (g/mol).
- Read Theoretical yield (g) directly beneath the three fields — it recalculates the instant any of them changes.
- Feed this result into this site's percent yield calculator as its Theoretical yield (g) input once you've measured your actual isolated mass, to see how efficiently the real reaction performed.
Worked example — 2 mol of limiting reagent forming carbon dioxide
Enter 2 into Limiting reagent (mol), 1 into Stoichiometric ratio (product : limiting reagent) for a reaction that produces one mole of carbon dioxide per mole of limiting reagent, and 44.01 into Product molar mass (g/mol) — CO2's standard published molar mass, from 12.011 for carbon plus 2 x 15.999 for oxygen. Theoretical yield reads 88.02 g.
That 88.02 g figure is the absolute ceiling for this reaction: no matter how carefully the synthesis is run, no more than 88.02 g of carbon dioxide can form from 2 mol of this limiting reagent at a 1:1 ratio. This site's percent yield calculator uses this exact 88.02 g as its Theoretical yield (g) input alongside a measured 74.817 g actual yield to report an 85.0% efficiency for the same reaction.
Questions
What is the limiting reagent, and why does it control the answer?
The limiting reagent is the reactant present in the smallest stoichiometric amount relative to what the balanced equation requires — it runs out first and stops the reaction from producing any more product, regardless of how much of the other reactants remain. Because it caps product formation, theoretical yield is always calculated from the limiting reagent's moles, never from an excess reagent's.
Where does the stoichiometric ratio come from?
Directly from the coefficients of your balanced chemical equation. If the equation reads 2 A + B -> 3 C, the stoichiometric ratio of product C to limiting reagent A is 3:2, or 1.5 moles of C per mole of A. Getting this ratio right requires the equation to actually be balanced first — an unbalanced equation gives a wrong ratio and a wrong theoretical yield no matter how carefully the rest of the arithmetic is done.
Does theoretical yield account for side reactions or impurities?
No — theoretical yield assumes a perfectly clean, complete reaction with no side products and no losses, which is exactly why it's called 'theoretical' rather than 'expected' or 'predicted actual.' Real reactions fall short of this ceiling for reasons theoretical yield deliberately ignores; that shortfall is what percent yield, calculated separately from a measured actual yield, is designed to quantify.
How is this different from the percent yield calculator?
This instrument computes the maximum possible product mass from pure stoichiometry — moles of limiting reagent, mole ratio, and molar mass — with no lab measurement involved. The percent yield calculator then takes that theoretical figure together with a measured actual yield and divides one by the other to report an efficiency percentage. Run this one first; its output feeds directly into that one.
What if my product molar mass is wrong?
The theoretical yield scales directly with whatever molar mass you enter, so an incorrect molar mass produces a proportionally incorrect theoretical yield with no warning from the instrument itself. Double-check the product's molecular formula and molar mass against a reliable reference — a periodic table and the formula's atom count — before relying on the result.