How to solve stoichiometry problems
- Write and balance the chemical equation.
- Convert each given mass to moles with its molar mass.
- Use the coefficients of the balanced equation as a mole ratio.
- When you know two reactant amounts, find the limiting reactant first.
- Convert the answer to grams, liters, or particles.
Stoichiometry problems with answers
These problems use H = 1.008, C = 12.011, N = 14.007, and O = 15.999. Use the calculator examples to check problems 1 to 3.
Problem 1
Mole to mole: N2 + 3 H2 → 2 NH3. How many moles of NH3 form from 3.0 mol of H2?
- The mole ratio is 2 mol NH3 : 3 mol H2.
- 3.0 mol H2 × (2 mol NH3 ÷ 3 mol H2) = 2.0 mol NH3.
Answer: 2.0 mol NH3
Problem 2
Gram to gram: CH4 + 2 O2 → CO2 + 2 H2O. How many grams of CO2 form when 10.0 g of CH4 burns completely?
- Moles of CH4 = 10.0 g ÷ 16.043 g/mol = 0.6233 mol.
- The mole ratio is 1 mol CO2 : 1 mol CH4, so 0.6233 mol CO2 forms.
- Mass of CO2 = 0.6233 mol × 44.009 g/mol = 27.4 g.
Answer: 27.4 g CO2
Problem 3
Limiting reactant: 25.0 g of N2 reacts with 5.00 g of H2 to make NH3. Which reactant is limiting, and how many grams of NH3 can form?
- N2 = 25.0 ÷ 28.014 = 0.8924 mol. H2 = 5.00 ÷ 2.016 = 2.480 mol.
- Divide by the coefficients: N2 = 0.8924 ÷ 1 = 0.8924, and H2 = 2.480 ÷ 3 = 0.8267.
- H2 gives the smaller value, so H2 is the limiting reactant.
- NH3 = 0.8267 × 2 = 1.653 mol. Mass = 1.653 mol × 17.031 g/mol = 28.16 g.
Answer: H2 is limiting. The maximum yield is 28.2 g NH3.
Problem 4
Percent yield: The reaction in problem 3 gives 22.5 g of NH3. What is the percent yield?
- Percent yield = actual yield ÷ theoretical yield × 100.
- Percent yield = 22.5 g ÷ 28.16 g × 100 = 79.9%.
Answer: 79.9%
Problem 5
Gas stoichiometry: 2 H2 + O2 → 2 H2O. What volume of O2 at 0 °C and 1 atm reacts with 4.00 g of H2?
- H2 = 4.00 g ÷ 2.016 g/mol = 1.984 mol.
- O2 = 1.984 mol ÷ 2 = 0.992 mol.
- At 0 °C and 1 atm, one mole of gas occupies 22.41 L.
- V = 0.992 mol × 22.41 L/mol = 22.2 L.
Answer: 22.2 L O2
How to use this calculator
- Enter the chemical equation. The calculator balances it.
- Enter a quantity for every reactant, such as N2=25.0 g, H2=5.00 g.
- Enter the product formula.
- Calculate to get the limiting reactant and the maximum product in moles and grams.
Formula and variables
reaction extent = nᵢ / nuᵢ
| Symbol | Meaning | Unit |
|---|---|---|
| nᵢ | species amount | mol |
| nuᵢ | balanced coefficient | dimensionless |
Worked example
For N2 + 3 H2 -> 2 NH3, 25.0 g N2 and 5.00 g H2 make H2 limiting. The maximum yield is 28.2 g NH3.
Interpretation
The smallest available stoichiometric extent identifies the limiting reactant; remaining material is computed from that same extent.
Common mistakes
- Supply one quantity for every reactant using the displayed formula labels.
- Mass quantities require atomic data for every element in the formula.
Frequently asked questions
How do you solve a stoichiometry problem?
Balance the equation, convert the given amount to moles, apply the mole ratio, and convert the answer to the required unit.
How do I check a problem with only one known reactant?
Enter the known reactant and a large excess of the other reactants. The known reactant then limits the product.
Can I mix grams and moles in one calculation?
Yes. The calculator converts each reactant quantity to moles before it compares reaction extents.
Does the calculator balance the equation first?
Yes. It uses exact integer coefficients before it applies any quantity ratio.
What does maximum product mean?
It is the theoretical product amount from the available reactants under complete conversion.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.