1. Write the balanced equation first

Before any number, write the reaction and balance it. The coefficients are the exchange rate between substances, and every mole ratio you use later comes from them. Most wrong stoichiometry answers are right arithmetic on an unbalanced equation. If the problem gives you the equation, check it anyway; if it does not, the balancing is the first half of the work.

2. List the givens with their units

Turn the sentence into a list: every quantity gets a symbol, a value and a unit, and the quantity you are asked for gets written at the bottom. Include what the words imply: "at STP" fixes temperature and pressure, "excess" tells you which reactant is not limiting, "1.0 M" means one mole per liter of solution.

3. Convert everything to moles

Moles are the currency the equation trades in. Grams become moles through molar mass, liters of solution through molarity, liters of gas through the ideal gas law or the molar volume at STP. Until every given is in moles, the coefficients cannot be used. The formula sheet lists each conversion with its conditions.

4. Use the mole ratio, then convert back

Multiply by the ratio of coefficients to get from the substance you know to the substance you want. If two reactants are given, find which runs out first: that is the limiting reactant, and it sets the maximum product. Then convert moles of the answer back into the unit that was asked for: grams, liters, molarity.

5. Check the answer before you believe it

Size: a few grams of reactant will not make a kilogram of product. Sign and direction: an exothermic reaction has a negative enthalpy change. Units: a molarity is mol/L, not mol. Significant figures: match the least precise given. Two seconds of checking catches most of what the method missed.

The method on a real problem

Problem. Ammonia is made by N₂(g) + 3H₂(g) → 2NH₃(g). A run starts with 25.0 g of N₂ and excess hydrogen, and isolates 24.0 g of ammonia. What is the percent yield?

  1. Hydrogen is in excess, so nitrogen is limiting. Convert it to moles: M(N₂) = 28.014 g/mol, so n = 25.0 / 28.014 = 0.89241 mol.
  2. Apply the mole ratio from the balanced equation, 2 mol NH₃ per 1 mol N₂: n(NH₃) = 2 × 0.89241 = 1.78482 mol.
  3. Convert to mass with M(NH₃) = 17.031 g/mol: theoretical yield = 1.78482 × 17.031 = 30.397 g, which is 30.4 g.
  4. Divide and scale: percent yield = (24.0 / 30.397) × 100% = 78.95%, which is 79.0% to three significant figures.

Answer. A theoretical yield of 30.4 g and a percent yield of 79.0%. The full guide is at Percent yield formula.

Questions people ask

Why do I understand the lesson but freeze on the problem?

Because knowing what a mole is and deciding to convert to moles under pressure are different skills, and only the second is practiced by doing problems. The method on this page makes the decision explicit: balance, list, convert, ratio, convert back. Freezing usually happens at the convert step, and it gets faster with repetition.

Do I need to memorize the formulas?

Memorize the conditions and the units more than the symbols. A formula sheet gives you the symbols; nothing on it tells you that the ideal gas law wants kelvin and consistent pressure units, or that molarity is moles per liter of solution, not of solvent. The formula pages on this site pair every formula with its conditions for that reason.

How many problems should I do?

Fewer than you think, done properly: equation balanced, every quantity carrying a unit, the answer checked for size and sign. Ten problems with the method beat forty done by matching the previous one.