Stoichiometry practice problems: follow the mole ratio

A four-step method

1. Balance the equation without changing chemical formulas.
2. If given mass, divide by molar mass to find moles.
3. Multiply by the coefficient ratio for the desired substance.
4. Convert the resulting moles to the requested units.

Coefficients compare amounts in moles, not equal masses. Write units on every line so the unwanted unit cancels. For background, see OpenStax reaction stoichiometry. The exercises here are independently written.

Worked example: ammonia from nitrogen

Use N₂ + 3H₂ → 2NH₃. Suppose 2.5 mol of nitrogen reacts with enough hydrogen. How much ammonia can form?

Calculation: 2.5 mol N₂ × (2 mol NH₃ ÷ 1 mol N₂) = 5.0 mol NH₃.

The nitrogen unit cancels. The factor of two comes from the balanced equation, not from the subscript in NH₃. Subscripts identify the atoms within a molecule; coefficients describe the reaction proportions. This calculation assumes complete reaction and sufficient hydrogen.

Worked example: find the limiting reactant

Now start with 2.0 mol N₂ and 3.0 mol H₂. Calculate possible ammonia from each reactant separately:

• Nitrogen could supply 2.0 × 2 = 4.0 mol NH₃.
• Hydrogen could supply 3.0 × 2/3 = 2.0 mol NH₃.

Hydrogen limits the reaction: only 2.0 mol NH₃ can form under these assumptions. Producing that amount consumes 1.0 mol N₂, leaving 1.0 mol N₂. The smaller starting number of moles is not automatically the limiting reactant; the coefficient ratio matters.

Try these chemistry practice questions

1. For N₂ + 3H₂ → 2NH₃, how much H₂ is needed for 4.0 mol NH₃?

2. For 2Mg + O₂ → 2MgO, start with 4.0 mol Mg and 1.5 mol O₂. Identify the limiting reactant and product amount.

3. Using a rounded molar mass of 24.3 g/mol, convert 48.6 g Mg to moles and determine MgO formed with excess oxygen.

Review your mistake, not just the answer

If your number is wrong, mark the first incorrect step: equation, mass conversion, ratio direction or final unit. Rewrite that step with units. Then change the starting amount and solve again without the model. Keep a separate card for why an excess reactant remains; a memorized arithmetic routine does not explain the chemical limit.

Write your own question pairs in the free flashcard maker, or use your chemistry notes to generate flashcards and check the answers against class material.

Practice specific topics

Choose the concept you want to review. Each lesson includes a worked example, common mistakes and three questions with answer explanations.

• Molarity practice problems
• Dilution calculations with M1V1 = M2V2
• Balancing chemical equations practice
• Ideal gas law practice problems
• pH and pOH practice problems
• Percent yield practice problems
• Empirical formula practice problems
• Oxidation number practice

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