Molecular Formula. Molecular formula and chemical symbols

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.
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

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