Chemical compounds are formed by more than one element coming together. The compounds so formed have a definite proportion of the elements with respect to one another.
This is because the valency of each atom is fixed and is to be satisfied by the other.As such while an element tries to satisfy its own valency, the other element too would satisfy its own valency while combining with this element. As a result, in a compound, irrespective of where they are found or exist, the ratio of the elements in that compound is fixed.For example let us consider the example of methane.Methane has one carbon atom combined with four hydrogen atoms.
Thus the ratio of carbon to hydrogen is 1:4. This ratio is universal on the back of earth. However certain compounds have the elements combined in such a ratio that the ratio can be expressed after dividing by a common devisor.For example consider the example of a compound C3H6O3 .As it can be observed the numbers of atoms of carbon are 3, that of hydrogen 6 and that of oxygen 3, the ratio is 3:6:3. The same ratio can also be expressed in a simplified way if we divide all these three numbers by the minimum common number that is 3. So the new ratio can be 1:2:1.
This is the empirical formula of C3H6O3. Thus empirical formula can be defined as a formula with the ratio of lowest whole numbers.Another example of empirical formula could be that of ethane.Ethane has the formula C2H6 .In this formula 2 is common so the ratio of the carbon to hydrogen atoms in ethane which is 2:6 can be expressed as 1:3, So C1:H3 is the empirical formula for ethane.Empirical Formula MassIn our examples we have seen that C3H6O3 has the molar mass of 3×12 + 6×1 + 16×3 = 90So the molar mass of C3H6O3 is 90. However as the empirical formula is C1H2O1 the empirical mass would be 12 + 2×1 + 16 = 30.
In other words the empirical formula mass can be defined as the sum of masses elements in the lowest ratio in a compound. Similarly the empirical formula mass of ethane would be 1×12 + 3×1 = 15. As against this the molar mass of ethane is 30.Usefulness of the empirical formula and empirical formula mass.