Definition: A mole ratio is ratio between the amounts in moles of any two compounds involved in a chemical reaction. Mole ratios are used as conversion factors between products and reactants in many chemistry problems.
Examples: For the reaction:
2 H2(g) + O2(g) → 2 H2O(g)
The mole ratio between O2 and H2O is 1:2. For every 1 mole of O2 used, 2 moles of H2O are formed.
The mole ratio between H2 and H2O is 1:1. For every two moles of H2 used, 2 moles of H2O is formed.