To get the molecular formula we use the individual atomic masses of the atoms making the molecule. Hence, to get the factor by which the empirical formula is multiplied, we divide molar mass by the total of the mass of the atoms making the empirical formula.
(C2H7) the mass of one carbon atoms is 12 so two atoms add up to 24. The mass of one hydrogen atom is 1 a.m.u thus 7 atoms give a total of 7 a.m.u. The sum of the two types of atoms is 24+7= 31
Molar mass=62.18
62.18/31=2.0
Hence, (C2H7) 2=C4H14
