MINIMUM FORMULA: HOW TO GET THE MINIMUM FORMULA, EXAMPLES AND EXERCISES - CHEMISTRY - 2024

The minimum or empirical formula is an expression of the proportion in which the atoms of the different elements that make up a chemical compound are present. The minimum formula is thus the simplest representation of a chemical compound. This is why it is 'minimal'.

The minimum formula does not express the numbers of atoms of the different elements that make up a chemical compound; but the proportion in which these atoms combine to generate the chemical compound.

This minimal formula just reveals that the compound has equal numbers of carbon and oxygen atoms, and twice as many hydrogen atoms. Source: Gabriel Bolívar.

The minimum formula is also called the empirical formula, since it is obtained from experimentation; that is, it is related to experience. Experimental observation allowed to conclude that two hydrogen atoms combine with one hydrogen atom to form water (H ₂ O).

In many cases, the minimum formula matches the molecular formula. In the molecular, the real numbers of the different elements that make up a chemical compound appear; such is the case, for example, of sulfuric acid (H ₂ SO ₄).

But in other cases, there is no coincidence of the minimum formula and the molecular formula; Such is the case of benzene: the minimum formula is CH, while its molecular formula is C ₆ H ₆.

How to get the minimum formula?

First step

Find the grams of each element present in the compound. As they are usually expressed as a percentage of the mass of the compound, a simple calculation must be made for the transformation of the mass percentages by mass of the elements.

Second step

The relative number of atoms of the different elements that make up the chemical compound is obtained. To do this, the mass of each element, expressed in grams, is divided by its atomic mass.

Some authors prefer to call the relative number of atoms, as the number of moles of atoms of the elements that are part of the chemical compound.

Third step

Obtaining the minimum proportion, expressed in whole numbers, of each of the atoms present in the chemical compound.

To find these values, divide the relative numbers of each atom present in the compound by the minimum relative numerical value that an atom has in the chemical compound.

In the event that the value of the minimum proportion of an atom is not a whole number without decimals, all the values ​​of the minimum proportion must be multiplied by a number; in such a way, that it allows all the values ​​of the minimum proportion to be whole numbers.

For example, if the value of the minimum proportion of an atom of the chemical compound is 2.5, all the values ​​of the minimum proportion of each of the atoms present must be multiplied by 2. Thus, all the values ​​of the minimum proportion will be only integer numbers.

Fourth step

Write the minimum formula of the compound. The values ​​of the minimum proportion for the atoms of the different elements will be placed as subscripts of the chemical symbols of elements to write the minimum formula.

Minimum formula examples

Water and carbon dioxide

Most chemical compounds have formulas that are both minimal and molecular formulas. Thus, the formula for water (H ₂ O) is a minimal formula; but it is also a molecular formula. The same goes for the formula for carbon dioxide (CO ₂).

Basic and acid oxides

Basic and acid oxides are chemical compounds that have chemical formulas, with a few exceptions that are both minimal and molecular formulas. The same situation occurs with hydroxides and acids, with the exception of some carboxylic acids.

Acetic acid

Carboxylic acids that have an even number of carbon atoms in their composition have, separately, a minimum formula and a molecular formula. Acetic acid, for example, has a molecular formula (C ₂ H ₄ O ₂); but it also has a minimal formula (CH ₂ O).

Butyric acid

Butyric acid also has a molecular formula (C ₄ H ₈ O ₂) and a minimal formula (C ₂ H ₄ O).

Ethylene glycol

Other organic compounds have both types of chemical formulas; such is the case of ethylene glycol, which has a molecular formula (C ₂ H ₆ O ₂) and a minimal formula (CH ₃ O).

Benzene

Benzene, an aromatic compound, has a molecular formula (C ₆ H ₆) and a minimal formula (CH).

Glucose

Glucose has a molecular formula (C ₆ H ₁₂ O ₆) and a minimal formula (CH ₂ O).

Solved exercises

- Exercise 1

Ethylene glycol is a compound used as antifreeze, it has a molecular weight of 62.1 g / mol and a percentage composition by mass as follows:

C: 38%, H: 9.7%

O: 51.16%.

First step

Express the mass percentages of the elements in their masses in grams. For this, the molecular weight of ethylene glycol is taken as a reference. If this is 62.1 grams per mole, 38% of it consists of carbon. And so on with the other elements:

Carbon mass = 62.1 g (38/100)

= 23.6 g

Mass of hydrogen = 62.1 g (9.7 / 100)

= 6.024 g

Mass of oxygen = 62.1 g (51.16 / 100)

= 31.77 g

Second step

Calculate the relative number of each atom present (NRA) or the number of moles of each atom. To do this, the mass of each chemical element is divided by its atomic mass.

NRA (C) = 23.6 g / 12 g

= 1.97

It can be rounded to 2.

NRA (H) = 6.024 g / 1 g

= 6.024

It can be rounded to 6.

NRA (O) = 31.77 g / 16 g

= 1,985

It can be rounded to 2.

Third step

Calculate the minimum whole number ratio of the elements of the compound (PMNE). To do this, divide each of the values ​​of the atomic relative numbers (NRA) by the smallest value of this. In this case, 2.

PMNE (C) = 2/2

= 1

PMNE (H) = 6/2

= 3

PMNE (O) = 2/2

= 1

Fourth step

Write the minimum formula for ethylene glycol:

CH ₃ O

As can be seen, the values ​​obtained in step 3 are the subscripts of the elements of the compound.

- Exercise 2

Acetic acid has a molecular weight of 60 g / mol and the following mass percentage composition:

C: 40%

H: 6.67%

Or: 53.33%

Obtain the minimum formula of the compound.

First step

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

Carbon mass = 60 g (40/100)

= 24 g

Mass of hydrogen = 60 g (6.67 / 100)

= 4 g

Mass of oxygen = 60 g (53.33 / 100)

= 32 g

Second step

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

NRA (C) = 24 g / 12 g

= 2

NRA (H) = 4 g / 1 g

= 4

NRA (O) = 32 g / 16 g

= 2

Third step

Obtain the minimum whole number ratio of the elements of the compound (PMNE). To do this, the NRA values ​​of the elements are divided by the lowest NRA value that an element has:

PMNE (C) = 2/2

= 1

PMNE (H) = 4/2

= 2

PMNE (O) = 2/2

= 1

Fourth step

Write the minimum formula for acetic acid:

CH ₂ O

The subscripts of the compound are the values ​​obtained from PMNE.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Helmenstine, Anne Marie, Ph.D. (August 27, 2019). Empirical Formula: Definition and Examples. Recovered from: thoughtco.com
3. Wikipedia. (2020). Empirical formula. Recovered from: en.wikipedia.org
4. Chemistry LibreTexts. (July 03, 2019). Calculating Empirical Formulas for Compounds. Recovered from: chem.libretexts.org
5. Nicola McDougal. (2020). Empirical Formula: Definition, Steps & Examples. Study. Recovered from: study.com