A mole ratio compares one substance to another on the basis of their molecular quantity. The ratio itself is a measure of the number of moles of one substance to that of another in the same system. This comparison may also be used to compare the number of moles present of one substance to that of the number of moles present in the entire system.
For example, the number of moles of sodium in an aqueous solution of sodium chloride compares the number of moles of sodium to the number of moles of sodium, chloride and water found within the aqueous solution.
The mass ratio is a comparison of the amount of mass of one element or chemical in the system to that of another or the entire system. This mass comparison is useful in material science as a determination of material composition. Furthermore, the mass ratio is commonly used to determine the molality of a solution with respect to the amount of solvent. This allows some reactions to be set up without the need for correction if the number of molecules present in a particular reaction mechanism changes.
The most common application of the mole ratio is to determine the required amounts of reagents needed in a particular reaction to produce a desired amount of product. Mole ratios allow for proportions to be used in a chemical reaction. For example, the reaction of two hydrogen atoms with one oxygen atom produces one water molecule. The mole ratio of hydrogen to water is 2 to 1 and therefore twice as many hydrogen atoms are needed than oxygen to produce one molecule of water.
Mass ratio is most commonly used in identifying the level of impurities after a chemical reaction. Any given chemical reaction has what is known as a theoretical yield. This is the maximum obtainable amount of product in any chemical reaction. After a particular reaction process is complete, the ratio of the amount of desired product, with respect to mass, against the mass of the produced material yields a percentage that tells chemists how close the result was to the maximum yield.