How to Approach Stoichiometry

Almost every stoichiometry problem follows the same path: get to moles, use the balanced equation to cross over to the substance you want, then get back to the units the question asks for.

The four-step road map

1. Balance the equation. The coefficients are the mole ratios you will use, so the equation has to be balanced first.
2. Convert the known amount to moles. From a mass, use n = m ÷ M. From a gas volume or a solution, use the relevant relationship (PV = nRT, or n = M × V).
3. Apply the mole ratio. Multiply by the ratio of coefficients from the balanced equation to get moles of the substance you want.
4. Convert to the requested units. Moles back to grams (m = n × M), to volume, or to particles (N = n × N_A), as needed.

Worked example

How many grams of FeCl₃ form when 38.5 g of Cl₂ reacts with excess iron, given 2 Fe + 3 Cl₂ → 2 FeCl₃?

Mole ratio Cl₂ → FeCl₃ is 2 : 3, so n(FeCl₃) = 0.543 × (2 ÷ 3) = 0.362 mol. Then m = 0.362 × 162.20 = 58.7 g of FeCl₃.

Common pitfalls

• Forgetting to balance first, so the mole ratio is wrong.
• Using the mass ratio instead of the mole ratio — only moles carry the coefficient ratio.
• Skipping unit conversions, especially mL to L and °C to K.

Start by finding molar masses with the Molar Mass Calculator, then handle solutions with the Molarity Calculator. When two reactants are given, see Limiting Reagents Explained.