MOLECULAR FORMULA: HOW TO GET IT, EXAMPLES AND EXERCISES - CHEMISTRY - 2024

The molecular formula is a representation of a molecule of a compound, in which the atoms of all the elements present in the compound and the number of them are indicated by their chemical symbols. Each atom is accompanied by a subscript.

The molecular formula has the limitation that it does not indicate the bonds of the different atoms of the molecule, nor their spatial relationship; just as the structural formula does. In some cases, it is not specific, for example: C ₆ H ₁₂ O ₆ is the molecular formula of glucose, galactose, and fructose.

Endless isomeric compounds can be obtained from the same molecular formula. This corresponds to a variable organic compound of high molecular mass. Source: Gabriel Bolívar.

There is a set of rules for the arrangement of atoms in a molecular formula. For ionic salts, for example sodium chloride, the cation symbol (Na ⁺) is placed first and then the anion symbol (Cl ^-). This is why the molecular formula of sodium chloride is NaCl.

The molecular formula of organic compounds is written by placing the symbol for carbon (C) first, followed by the symbol for hydrogen (H), and then the remaining atoms. For example, the molecular formula of acetic acid is C ₂ H ₄ O ₂.

How to get the molecular formula

Obtaining the molecular formula of a compound is based on the prior establishment of its minimum or empirical formula. For this reason, the initial steps in obtaining both formulas are common.

First step

It is usual to express the mass of the elements of a chemical compound in a percentage form. A simple calculation must then be made to transform the mass percentages of the different elements into their masses expressed in grams.

Second step

Knowing the mass in gram of each of the elements, and their atomic mass, the relative number of each atom or the number of moles of each atom can be obtained by dividing the mass of each of the different elements by their atomic mass.

Third step

Divide the relative numbers of all the atoms by the relative number with the smallest value. The values ​​of the minimum proportion must be whole numbers. In the case that a value of the proportion contains a decimal, a correction must be made to make that decimal disappear.

The correction consists of multiplying all the values ​​of the proportion by a number that returns all the values ​​to integers. For example, if a value of the minimum proportion is 1.25, all the values ​​must be multiplied by four (4). Since 1.25 multiplied by 4 equals 5, a whole number.

Fourth step

All the chemical symbols of the elements of the compound will be placed and with their corresponding subscript; that is, the value of the minimum proportion of the element.

Fifth step

The minimum formula of a compound can be transformed into its molecular formula by obtaining the ratio between the molecular weight of the compound and the weight of its minimum formula (n). This relationship is the same as that between the molecular formula and the minimum formula.

n = molecular weight of compound / weight of minimum formula

To obtain the molecular formula, multiply the minimum formula by n. This implies that each one of the subscripts of the minimum formula must be multiplied by n.

n = Molecular weight / Minimum formula weight

= Molecular formula / Minimum formula

Molecular formula = nminimum formula

Examples of molecular formulas

Some common molecular formulas are listed below:

-Ferrous oxide (FeO)

-Sodium hydroxide (NaOH)

-Potassium oxide (K ₂ O)

-Perchloric oxide (Cl ₂ O ₇)

-Ferric hydroxide

-Calcium chloride (CaCl ₂)

-Hydrochloric acid (HCl)

-Sodium sulfate (N ₂ SO ₄)

-Sulfuric acid (H ₂ SO ₄)

-Perbromic acid (HBrO ₄)

-Aluminum chloride (AlCl ₃)

-Methane (CH ₄)

-Propane (C ₃ H ₈)

-Ethane (C ₂ H ₆)

-Ethyl alcohol (C ₂ H ₆ O)

-Acetic acid (C ₂ H ₄ O ₂)

-Benzene (C ₆ H ₆)

-Glucose (C ₆ H ₁₂ O ₆)

Solved exercises

- Exercise 1

Methyl benzoate has the following mass percentages:

C: 70.57%, H: 5.93%

Or: 23.49%

And a molecular weight of 136.1 g / mol. Determine the molecular formula.

First step

The first step is to express the mass percentages of the elements in their masses in grams, knowing that the mass percentages are in relation to the molecular weight.

Of the 136.1 grams corresponding to the mole of the compound, 70.57% represents only carbon. And so on with the remaining elements:

Carbon mass = 136.1 g (70.57 / 100)

= 96.05 g

Mass of hydrogen = 136.1 g (5.93 / 100)

= 8.06 g

Mass of oxygen = 136.1 (23.49 / 100)

= 31.96 g

Second step

In the elaboration of the molecular formula, the relative numbers of atoms (NRA) must be determined. To do this, the masses of the elements are divided by their atomic masses:

NRA (C) = 96 g / 12 g

= 8

NRA (H) = 8 g / 1 g

= 8 g

NRA (O) = 32 g / 16 g

= 2

Third step

We must then calculate the minimum proportions of the elements (PME). To do this, all the NRA values ​​are divided by the lowest NRA value:

PME (C) = 8/2

= 4

PME (H) = 8/2

= 4

PME (O) = 2/2

= 1

Fourth step

Write the minimum formula for methyl benzoate, based on the fact that the subscripts of the elements are their calculated minimum proportions:

C ₄ H ₄ O

Fifth step

Finally, the molecular formula of methyl benzoate must be established. We therefore calculate n:

n = molecular weight / minimum formula weight

The weight of the minimum formula is 68 g / mol:

= 136.1 g / 68 g

n ≈ 2

Molecular formula = 2minimum formula

= 2 C ₄ H ₄ O

Molecular formula of methyl benzoate = C ₈ H ₈ O ₂

- Exercise 2

The percentages of the elements that make up benzene are as follows:

C: 92.36%

H: 7.64%.

The molecular weight of benzene is 78 g / mol. Determine its molecular formula.

First step

Transform the mass percentages of the elements into their masses in grams:

Carbon mass = 78 g (92.36 / 100)

= 72.04 g

Mass of hydrogen = 78 g (7.64 / 100)

= 5.96 g

Second step

Calculation of the relative numbers of atoms (NRA). To do this, the masses of the elements are divided by their atomic masses:

NRA (C) = 72 g / 12 g

= 6

NRA (H) = 6 g / 1 g

= 6

Third step

Calculation of the values ​​of the minimum proportion of the elements (PME). This is calculated by dividing the NRA values ​​of all items by the smallest NRA value:

PME (C) = 6/6

= 1

PME (H) = 6/6

= 1

Fourth step

Write the minimum formula for benzene, taking into account that the subscripts of the elements are the values ​​of the minimum proportion calculated.

Minimum Benzene Formula: CH

Fifth step

Establishment of the molecular formula by calculating the relationship between the molecular mass and the mass of the minimum formula:

n = Molecular weight / Weight of the minimum formula

= 78 g / 13 g

n = 6

Molecular formula = nminimum formula

= 6CH

= C ₆ H ₆

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Wikipedia. (2020). Chemical formula. Recovered from: en.wikipedia.org
3. Boundless Learning. (sf). Molecular Formulas. Recovered from: courses.lumenlearning.com
4. Helmenstine, Anne Marie, Ph.D. (September 29, 2019). Calculate Empirical and Molecular Formulas. Recovered from: thoughtco.com
5. Peter J. Mikulecky, Chris Hren. (2020). How to Use Empirical Formulas to Find Molecular Formulas. Recovered from: dummies.com