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Chemistry: Atoms to AP Chemistry · Module 4 of 15

The Mole & Stoichiometry

Counting atoms and molecules by the mole, and using balanced equations to relate amounts of reactants and products.

In this module
  1. The Mole Concept & Molar Mass
  2. Percent Composition & Empirical/Molecular Formulas
  3. Stoichiometric Calculations
  4. Limiting Reactants & Percent Yield
  5. Check yourself

The Mole Concept & Molar Mass

  • A mole is 6.022 × 10²³ particles (Avogadro's number); molar mass (g/mol) is numerically equal to an element's atomic mass or a compound's sum of atomic masses.
  • Moles connect the atomic scale (individual atoms or molecules) to the lab scale (grams you can actually measure).
Worked example

What is the molar mass of water, H₂O (H ≈ 1.01, O ≈ 16.00)?

Solution. 2(1.01) + 16.00 = 2.02 + 16.00 = 18.02 g/mol.

Percent Composition & Empirical/Molecular Formulas

  • Percent composition = (mass of an element in the compound ÷ total molar mass) × 100%.
  • The empirical formula shows the simplest whole-number ratio of atoms; the molecular formula is a whole-number multiple of the empirical formula.
Worked example

A compound has an empirical formula of CH₂ and a molar mass of 42 g/mol. What is its molecular formula?

Solution. Empirical formula mass = 12 + 2(1) = 14 g/mol. 42 ÷ 14 = 3. Molecular formula = (CH₂) × 3 = C₃H₆.

Stoichiometric Calculations

  • Use the balanced equation's coefficients as mole ratios to convert between moles of one substance and moles of another.
  • General path: grams (given) → moles (using molar mass) → moles (using the mole ratio) → grams (of what's asked, using molar mass).
Worked example

Using 2H₂ + O₂ → 2H₂O, how many moles of H₂O form from 4 moles of H₂ (with excess O₂)?

Solution. The mole ratio of H₂ to H₂O is 2:2, or 1:1, so 4 moles of H₂ produces 4 moles of H₂O.

Limiting Reactants & Percent Yield

  • The limiting reactant runs out first and determines the maximum ("theoretical") amount of product; the other reactant is left in excess.
  • Percent yield = (actual yield ÷ theoretical yield) × 100%, comparing what was really produced in the lab to what the math predicts.
Worked example

A reaction's theoretical yield is 20.0 g, but only 16.0 g is actually collected. What is the percent yield?

Solution. (16.0 ÷ 20.0) × 100% = 80%.

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. How many particles are in one mole of a substance?

2. What is the molar mass of CO₂ (C ≈ 12.01, O ≈ 16.00)?

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