Limiting Reactant Calculator

How it is calculated

For two reactants A and B, the tool divides the moles of each by its coefficient in the balanced equation: ratio A = nA ÷ νA and ratio B = nB ÷ νB. Here n is the amount in moles and ν is the stoichiometric coefficient. The smaller ratio belongs to the limiting reactant, because it supports fewer "reaction turns" before it runs out. Both ratios are shown to four decimals. Coefficients must be greater than zero and moles cannot be negative, otherwise nothing is returned. If the two ratios are equal, the reactants are in exact stoichiometric proportion, but the tool still labels the result "Reactant A". It does not convert grams to moles, so do that first with the molar mass.

Common mistakes

• Entering grams instead of moles. The tool compares mole amounts, so convert each mass by dividing by the molar mass first.
• Comparing the raw moles instead of the ratios. In the example, 2 mol of B is fewer than 3 mol of A, but A is the limiting one because it needs twice as many moles per reaction turn.
• Using the coefficients from an unbalanced equation. If the equation is not balanced, the ratios and the answer are wrong.
• Reading a tie as "A is limiting". When both ratios are equal, such as 4 mol of A with coefficient 2 and 2 mol of B with coefficient 1, both reactants run out together and nothing is left over, even though the tool prints Reactant A.
• Assuming the result gives the excess left over. The tool only names the limiting reactant and the two ratios; you must work out the leftover amount and the yield yourself.