Chemistry · General chemistry I · Concept
Limiting reactant and percent yield
The limiting reactant is the one that runs out first, so it sets how much product can form. To find it, work out how much product each reactant could make on its own: the smaller amount is the theoretical yield, and actual ÷ theoretical × 100% is the percent yield.
Why one reactant limits
A balanced equation fixes the ratio in which reactants combine. If you mix them in any other ratio, one runs out while some of the other is left over. The one that runs out first is the limiting reactant; the leftover is the excess reactant.
How do you find the limiting reactant?
Convert each reactant to moles, then use the mole ratio to find how much product each could make if the other were unlimited. The reactant that gives less product is limiting. Comparing grams of reactants directly does not work, because the reactants have different molar masses and coefficients.
An equivalent shortcut
Divide each reactant’s amount in moles, n, by its coefficient, ν. The smallest quotient belongs to the limiting reactant; it is the extent of reaction, the number of moles of the reaction as written that can occur.
Theoretical, actual and percent yield
The theoretical yield is the product the limiting reactant can make if the reaction goes to completion with no losses. The actual yield is what you measure. Percent yield compares them; a value above 100% signals an impure or wet product, or a measurement error.
How much excess is left?
Use the limiting reactant and the mole ratio to find how much of the excess reactant is consumed, then subtract that from the amount you started with.
Common mistakes
- Choosing the reactant with the smaller mass: in the worked example there is less aluminum than chlorine by mass, yet chlorine limits.
- Skipping the coefficients when comparing moles: 0.37 mol Al and 0.49 mol Cl₂ do not mean chlorine is in excess.
- Using the excess reactant to calculate the theoretical yield.
- Putting the theoretical yield in the numerator of the percent yield.
Key terms
- Limiting reactant
- The reactant that runs out first. It sets the most product the reaction can make; the other reactants are left over, in excess.
- Excess reactant
- A reactant that is not used up when the limiting reactant runs out. How much is left over comes from the stoichiometry, not from comparing starting masses.
- Theoretical yield
- The most product a reaction can make, calculated from the limiting reactant, assuming the reaction goes to completion with no losses or side reactions.
- Actual yield
- The amount of product actually obtained from a reaction. It is usually less than the theoretical yield because of side reactions, incomplete reaction or losses during recovery.
- Percent yield
- percent yield = (actual yield ÷ theoretical yield) × 100%. Both must be for the same product and in the same units.
- Mole ratio
- A ratio of moles of two substances taken from the coefficients of a balanced equation, such as 2 mol H₂ : 1 mol O₂. It converts moles of one substance into moles of another.
Work through an example
Aluminum reacts with chlorine: 2Al(s) + 3Cl₂(g) → 2AlCl₃(s). A mixture of 10.0 g Al and 35.0 g Cl₂ produces 38.0 g AlCl₃. Which reactant limits, what is the theoretical yield, and what is the percent yield?
Find the limiting reactant from two masses →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
Open the example inputs, change a value and keep a useful result on your board.
Open the mixture in Stoichiometry & yield Check the chains in a Chemistry box Open worked example on a board Chemistry formulas: matter and amountYour existing work stays on this device. Examples open as editable copies.