When you compare amounts of chemicals as they relate to each other, you need the mole ratio. A mole is 6.022 x 10^23, or Avogadro's number, molecules or atoms of any substance. The mole ratio compares the number of moles of one substance in a chemical equation with another substance, usually on the opposite side of the equation. Stoichiometry is an area in chemistry that deals with amounts of chemicals in reactions and therefore uses the mole ratio extensively.
Determine which elements or compounds are in your reaction and write the skeleton equation. The equation in this example is the burning of propane to get carbon dioxide and water.
C3H8 + O2 ---> CO2 + H2O
Determine the coefficients necessary to balance the number of atoms on each side of the equation. In this example, you have three carbon atoms on the left side and one carbon atom on the right, so give the CO2 term a coefficient of 3. Do the same calculation for the hydrogen atoms. Water then gets a coefficient of 4.
C3H8 + ? O2 ---> 3 CO2 + 4 H2O
Notice that all the terms on the right side now have coefficients. Use them to calculate the number of O2 molecules needed on the left side.
C3H8 + 5 O2 ---> 3 CO2 + 4 H2O
Calculate the mole ratio. Start with the quantities given to you in the problem. A typical question might be, "How much propane must be burned to get a gallon of water?" Water, in this problem, is the known quantity, so the coefficient of the water term in the equation goes into the denominator (bottom) of the ratio. The coefficient for the unknown quantity--propane in this example--goes into the numerator. For every mole of propane burned in this example, you get 4 moles of water.
Mole ratio = 1/4