Stoichiometry Calculator

About Stoichiometry Calculator

Stoichiometry uses balanced equation coefficients as mole ratios to calculate amounts of reactants and products.

How to use this calculator

1. Enter the known amount for one substance in the reaction.
2. Choose the substance you want to find.
3. Use the balanced equation coefficients to set up the mole ratio with \(\frac{n_A}{a} = \frac{n_B}{b}\).
4. Calculate the unknown amount, then check that your units match what you want, such as moles, grams, or liters.

The formula explained

The formula \(\frac{n_A}{a} = \frac{n_B}{b}\) compares the amounts of two substances using their coefficients from a balanced chemical equation. It works because balanced equations preserve the correct mole ratio between reactants and products.

• \(n_A\) = the amount in moles of substance A
• a = the coefficient of substance A in the balanced equation
• \(n_B\) = the amount in moles of substance B
• b = the coefficient of substance B in the balanced equation

Step by step method

1. Write the balanced chemical equation and identify the coefficients for the two substances.
2. Set up the proportion \(\frac{n_A}{a} = \frac{n_B}{b}\) using the known amount and the unknown amount.
3. Cross-multiply and solve for the unknown number of moles.

Worked example

Problem. In the reaction \(2H_2 + O_2 \rightarrow 2H_2O\), how many moles of \(H_2O\) form from \(3\) moles of \(H_2\)?

1. From the balanced equation, \(a = 2\) for \(H_2\) and \(b = 2\) for \(H_2O\).
2. Set up the proportion \(\frac{3}{2} = \frac{n_{H_2O}}{2}\).
3. Solve: \(n_{H_2O} = 3\).

Tips and common mistakes

• Always use a balanced equation first, because the coefficients are what make the mole ratio correct.
• If your answer seems too large or too small, check whether you used the right coefficients and whether you converted units correctly.

Frequently asked questions