Mass to Mass Stoichiometry

A mole ratio is the conversion factor that shows the relationship of moles of any two substances in a chemical reaction. The mole ratio comes from the coefficients of the balanced equation. The coefficients can represent moles or molecules, but cannot represent grams. Substances in a reaction react on a mole to mole basis and not gram to gram.

This is best viewed in the reaction between hydrogen and oxygen to form water.

Two moles of hydrogen and one mole of oxygen react to form two moles of water. Two grams of hydrogen and one gram of oxygen will not produce two grams of water. Notice that moles of reactant do not equal moles of product. This is because each substance has a different number of grams in a mole. In order to relate grams of one substance to grams of another substance, each must be converted to moles and then the mole ratio is used for comparison.