The Limiting Reactant

General Chemistry • Chemical Reactions

Written by STEM Calculators Team Published October 17, 2025 Updated February 24, 2026

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Limiting Reactant and Product Amounts

After you balance a reaction, the coefficients give the mole ratios. When masses of all reactants are known, convert each mass to moles, compare how many “reaction blocks” each can supply, and the smallest one determines the limiting reactant. Use that to compute amounts of products and any excess reactant remaining.

Workflow

Balance the equation.
Convert reactant masses to moles.

\[ n_i = \frac{m_i}{M_i} \]

Compare reaction extents. Divide each reactant’s moles by its coefficient.

\[ \xi_i = \frac{n_i}{\nu_i}, \qquad \xi^{\ast}=\min(\xi_i) \]

Amounts at completion. Use \(\xi^{\ast}\) with coefficients.

\[ n^{\text{cons}}_{\text{react},i}=\nu_i\,\xi^{\ast}, \qquad n^{\text{left}}_{\text{react},i}=n_i-\nu_i\,\xi^{\ast}, \qquad n^{\text{prod}}_{\text{prod},j}=\nu_j\,\xi^{\ast} \]

Convert moles to grams.

\[ m = n\,M \]

Worked sketch

Example reaction: \(\mathrm{P_4 + 6\,Cl_2 \rightarrow 4\,PCl_3}\) with \(m(\mathrm{P_4})=125\ \text{g}\) and \(m(\mathrm{Cl_2})=323\ \text{g}\).

1) Convert masses to moles

\[ n(\mathrm{P_4}) = \frac{125}{123.9} = 1.01\ \text{mol} \] \[ n(\mathrm{Cl_2}) = \frac{323}{70.91} = 4.56\ \text{mol} \]

2) Reaction extent and limiting reactant

\[ \xi_{\mathrm{P_4}} = \frac{1.01}{1} = 1.01 \qquad \xi_{\mathrm{Cl_2}} = \frac{4.56}{6} = 0.760 \] \[ \xi^{\ast} = 0.760 \quad\Rightarrow\quad \text{limiting reactant: } \mathrm{Cl_2} \]

3) Amounts of products

\[ n(\mathrm{PCl_3}) = 4\,\xi^{\ast} = 4(0.760) = 3.04\ \text{mol} \]

4) Convert to mass

\[ m(\mathrm{PCl_3}) = 3.04 \times 137.3 \approx 4.17\times10^{2}\ \text{g} \]

Frequently Asked Questions

What is the limiting reactant and why does it matter?

The limiting reactant is the reactant that runs out first when the reaction goes to completion. It determines the maximum possible amount of products that can form.

How does the calculator decide which reactant is limiting?

It converts each reactant mass to moles and divides by its balanced coefficient. The smallest n / coefficient value indicates the limiting reactant.

Does the calculator show how much excess reactant is left over?

Yes. After finding the limiting reactant, it computes how many moles of each reactant are consumed and subtracts from the starting moles to report leftover excess in both moles and grams.

Which formulas are used in limiting reactant calculations?

Key steps include n = m / M to convert grams to moles, comparing n / coefficient to find the limiting reactant, and m = n x M to convert final moles back to grams.

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