Mole ratio from a balanced equation
The coefficients in a balanced equation give the ratio of moles reacting. Use that ratio as a conversion factor to go from the moles of one substance to the moles of another. Atoms are conserved, so both sides must balance.
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Analogy
A recipe: 'two eggs for every cup of flour' works for any batch size.
Example
3.0 mol H₂ makes how much NH₃?Ratio 2 NH₃ : 3 H₂3.0 × (2/3) = 2.0 mol NH₃
In practice
A pharmacist or a chemical plant uses mole ratios to work out how much raw material is needed for a target amount of product.
Practice
Flashcards
- How do you convert between substances in an equation?Use the mole ratio from the balanced coefficients.
Interview questions
- Calculate the mass of NH₃ produced from 28.0 g of N₂ with excess H₂ (N₂ + 3H₂ → 2NH₃).
- Moles N₂ = 28.0 / 28.02 ≈ 1.00
- Ratio 1 N₂ : 2 NH₃
- Moles NH₃ = 2.00
- Mass = 2.00 × 17.03 ≈ 34.1 g
Questions people ask
Why must an equation be balanced?
The law of conservation of mass says atoms are not created or destroyed in a chemical reaction. So the same number of each type of atom must appear on both sides. Balancing by changing coefficients, never subscripts, makes the equation obey this law and give correct mole ratios.
Can I use grams directly in the mole ratio?
No. Coefficients count particles, not mass. Always convert grams to moles first, use the mole ratio, then convert back to grams if needed. The path is grams of A, then moles of A, then moles of B, then grams of B. Skipping the mole step is the most common mistake in stoichiometry.