Chapter 8 · Quantities in Chemical Reactions
8.3Making Molecules: Mole-to-Mole Conversions
10 min · two checks
Predict
How many moles of H₂O form from 3.0 mol of H₂ in 2 H₂ + O₂ → 2 H₂O, if oxygen is plentiful?
The idea
- Convert moles of one substance to moles of another with a coefficient ratio.
- Write the ratio so units cancel.
The conversion factor is (coefficient of wanted) / (coefficient of given), in moles. From the water equation, mol H₂O = mol H₂ × (2 mol H₂O / 2 mol H₂). From combustion of methane, CH₄ + 2 O₂ → CO₂ + 2 H₂O, moles of oxygen required are twice the moles of methane. Write the factor explicitly. A bare multiplication by 2, with no labels, will be backwards half the time.
This step assumes the other reactant is available in excess, or that you were handed moles of the reactant you intend to run to completion. If both reactants are limited, you cannot convert from either one until you know which one runs out. That is the next major idea. Mole-to-mole is still the center of the map either way.
Keep these
- Use coefficients as a mol-to-mol factor.
- The factor is wanted over given.
- This step assumes you know which reactant is fully used.
Worked path
How many moles of O₂ are needed to burn 0.80 mol of C₃H₈ completely? C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.
Observe
Given 0.80 mol C₃H₈. Want mol O₂.