Most stoichiometry homework help questions come down to the same issue: students know the individual pieces (moles, molar mass, balanced equations) but haven't practiced chaining them together in the right order. Once you have a consistent method, stoichiometry problems become repetitive in a good way — the same steps, different numbers.

The step-by-step method for mole calculations

  1. Write and balance the chemical equation. Every stoichiometry problem starts here — an unbalanced equation gives you wrong mole ratios for every step after.
  2. Convert what you're given into moles. If you're given grams, divide by molar mass. If you're given liters of gas at STP, divide by 22.4 L/mol.
  3. Use the mole ratio from the balanced equation to convert moles of what you're given into moles of what you want to find.
  4. Convert moles of your answer back into the units asked for (grams, liters, molecules, etc.).
  5. Check whether a limiting reactant is involved. If two reactants are given, you likely need to find the limiting reactant before finishing the problem.

Worked example

How many grams of water form when 10 grams of hydrogen gas react completely with oxygen? (2H₂ + O₂ → 2H₂O)

Step 1: Equation is already balanced.

Step 2: Convert 10 g H₂ to moles: 10 g ÷ 2 g/mol = 5 mol H₂

Step 3: Mole ratio of H₂ to H₂O is 2:2 (i.e. 1:1), so 5 mol H₂ produces 5 mol H₂O.

Step 4: Convert moles of H₂O to grams: 5 mol × 18 g/mol = 90 g H₂O

Common mistakes

Skipping the balanced equation step. Using an unbalanced equation gives you the wrong mole ratio, which throws off every step that follows — always balance first, before any chemistry calculations.

Forgetting to convert back to the requested units. Many students do all the mole-ratio work correctly, then submit an answer still in moles when the question asked for grams.

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Related reading

Looking for more college chemistry help? This method applies to almost any stoichiometry problem you'll see in an intro course.