Stoichiometry: Mole Ratios and Mass-to-Mass Steps
Convert between grams and moles and use the mole ratio from a balanced equation, with two worked chemistry examples.
The road map
- grams A → moles A (divide by molar mass)
- moles A → moles B (mole ratio from the equation)
- moles B → grams B (multiply by molar mass)
The balanced equation gives the mole ratio.
Example 1
N₂ + 3H₂ → 2NH₃. How many moles of NH₃ form from 6.0 mol H₂?
- Mole ratio: 2 mol NH₃ per 3 mol H₂
- 6.0 mol H₂ × (2 / 3)
4.0 mol NH₃
Example 2
How many grams of NH₃ form from 28 g of N₂? (N₂ = 28 g/mol, NH₃ = 17.03 g/mol)
- 28 g ÷ 28 g/mol = 1.0 mol N₂
- Mole ratio: 1 mol N₂ makes 2 mol NH₃
- 2.0 mol × 17.03 g/mol
34 g NH₃
Common mistake
Use the mass ratio from the equation
Coefficients are mole ratios, not mass ratios.
Balance the equation before you start.
Always convert grams to moles first.
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