Molecular formula is the combination of chemical symbols and indices that express the real numbers of atoms of each element present in a molecule.
To determine the molecular formula, it is first necessary to obtain the empirical formula, which is the formula that shows the relative numbers of atoms of each element in a compound. It expresses the proportion of atoms; for example, in glucose the ratio is one carbon atom, to two hydrogen, to one oxygen, that is, CH2O (1:2:1). The next step is to calculate the mass from this empirical formula. Knowing that the atomic masses of these elements are equal to: C = 12, H = 1 and O=16.
From the empirical formula (CH2O), we calculate the mass of this formula by multiplying the atomic masses by their respective masses:
C = 12. 1= 12
H = 1.2 = 2
O =16. 1 = 16__________
CH mass2O = 30
But the empirical formula does not indicate by itself what the molecular formula will be, after all this ratio of 1:2:1 is given in all the following cases: CH2O (preservative in formalin solution), C2H4O2 (vinegar acetic acid), C 3H6O3 (lactic acid), among others.
So, another important data we need to know is the molar mass of this compound, which is usually determined by a mass spectrometer.
Mass spectrometer device used to measure the molar mass of substances.
In the case of glucose, its molar mass is 180 g/mol. Then, we can calculate how many times the mass of the empirical formula “fits” in the molecular mass of the substance:
180/30 = 6
Multiplying the proportions in which each element appears in the molecule by 6, we have the proportion of each element in the molecular formula:
C = 1. 6= 6
H = 2.6=12
O=1. 6= 6
With the defined ratio (6:12:6), we have the molecular formula for glucose: Ç6H12O6.
By Jennifer Fogaça
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/formula-molecular.htm