Limiting Reactant Calculator

About the Limiting Reactant Calculator

For a reaction aA + bB → products, the limiting reactant is the one with the smallest mole-to-coefficient ratio. It is fully consumed and determines the maximum product yield.

How to use this calculator

1. Enter the moles of each reactant, such as \(n_A\) and \(n_B\).
2. Enter the stoichiometric coefficients from the balanced equation, such as \(a\) and \(b\).
3. Compare \(\frac{n_A}{a}\) and \(\frac{n_B}{b}\).
4. The smaller value identifies the limiting reactant, and the larger value shows the excess reactant.

The formula explained

The comparison \(\frac{n_A}{a}\) versus \(\frac{n_B}{b}\) tells you how many reaction portions each reactant can supply. The reactant with the smaller ratio is the limiting reactant because it is used up first.

• \(n_A\) = moles of reactant A
• a = stoichiometric coefficient of reactant A
• \(n_B\) = moles of reactant B
• b = stoichiometric coefficient of reactant B

Step by step method

1. Write the balanced chemical equation and note each reactant's coefficient.
2. Divide the moles of each reactant by its coefficient, so you get \(\frac{n_A}{a}\) and \(\frac{n_B}{b}\).
3. Compare the two values, and the smaller one tells you the limiting reactant.

Worked example

Problem. For the reaction \(2\,\text{H}_2 + 1\,\text{O}_2 \rightarrow 2\,\text{H}_2\text{O}\), you have \(5.0\) mol of \(\text{H}_2\) and \(2.0\) mol of \(\text{O}_2\). Find the limiting reactant.

1. Compute \(\frac{5.0}{2} = 2.5\) for \(\text{H}_2\).
2. Compute \(\frac{2.0}{1} = 2.0\) for \(\text{O}_2\).
3. Because \(2.0 < 2.5\), \(\text{O}_2\) is the limiting reactant.

Tips and common mistakes

• Always use the balanced equation, because the coefficients control the comparison.
• A smaller number of moles does not always mean the limiting reactant, since the coefficients can change the result.

Frequently asked questions