A clear method for mass–mole conversions and reaction yields using molar mass and balanced equations.
Why Moles Matter
Chemical reactions are governed by particle ratios, not gram ratios. A balanced equation such as
2 H₂ + O₂ → 2 H₂O
states that 2 moles of H₂ react with 1 mole of O₂. Equal masses of H₂ and O₂ do not give a 1:1 particle ratio because their molar masses differ.
Standard Conversion Path
For most stoichiometry problems:
- Mass (g) → divide by molar mass → moles
- Apply the mole ratio from the balanced equation
- Moles → multiply by molar mass → mass (g) (or convert to volume/particles as needed)
Molar Mass
Molar mass (M) is the mass of one mole of substance, in g/mol. Add atomic masses from the periodic table:
- C = 12.01 g/mol
- O = 16.00 g/mol
- CO₂ = 12.01 + 2(16.00) = 44.01 g/mol
Core Formula
n = \frac{m}{M}- n = amount of substance (mol)
- m = mass (g)
- M = molar mass (g/mol)
Worked Example
How many grams of water form when 4.00 g of H₂ reacts completely with excess O₂?
- Moles of H₂: $n = 4.00 / 2.016 ≈ 1.98$ mol
- Mole ratio: 2 mol H₂ → 2 mol H₂O, so n_{\mathrm{H_2O}} \approx 1.98 mol
- Mass of H₂O: $m = 1.98 × 18.02 ≈ 35.7$ g
Limiting Reactant & Yield
When both reactants are limited, convert each to moles of product; the smaller result is the theoretical yield. Percent yield is:
\%\ \mathrm{yield} = \frac{\mathrm{actual\ yield}}{\mathrm{theoretical\ yield}} \times 100\%Practice Calculator
Convert between grams and moles, or check reaction stoichiometry:
Interactive Calculator
Full tool: Stoichiometry Calculator. Reference: Polyatomic Ions Cheatsheet.
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