Define empirical formula and molecular formula. How will you establish a relationship between the two? Give examples.
The empirical formula of a compound expresses the simplest whole-number ratio of the atoms of the various elements present in one molecule of the compound For example, the empirical formula of benzene is CH and that of glucose is CH2O. This suggests that in the molecule of benzene one atom of Carbon (C) is present for every atom of Hydrogen (H). Similarly in the molecule of glucose (CH2O), for every one atom of C, there are two atoms of H and one atom of O present in its molecule. Thus, the empirical formula of a compound represents only the atomic ratio of various elements present in its molecule. The molecular formula of a compound represents the true formula of its molecule. It expresses the actual number of atoms of various elements present in one molecule of a compound. For example, the molecular formula of benzene is C6H6 and that of glucose is C6H12O6. This suggests that in one molecule of benzene, six atoms of C and 6 atoms of H are present. Similarly, one molecule of glucose (C6H12O6) actually contains 6 atoms of C, 12 atoms of H, and 6 atoms of O. Relation between the empirical and molecular formula Molecular formula = n × Empirical formula where n is an integer such as 1, 2, 3… When n = 1; Molecular formula = Empirical formula When n = 2; Molecular formula = 2 × Empirical formula. The value of n can be obtained from the relation. n = Molecular mass/ Empirical formula mass The molecular mass of a volatile substance can be determined by Victor Meyer’s method or by employing the relation. Molecular mass = 2 × vapour density . Empirical formula mass can however be obtained from its empirical formula simply by adding the atomic masses of the various atoms present in it. Thus the empirical formula mass of glucose CH20 = 1 × 12 + 2 × 1 + 1 × 16 = 30.0 u.
