Law of Constant Proportion

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Law of Constant Proportion

To explain the law of constant proportion, it is important to know the law of definite proportions definition. This is what the law states.


The law of constant composition states that the chemical compounds consist of the elements which are present in a fixed ratio by their mass. This means that any given pure sample of the compound, irrespective of its source, will always contain the same kind of elements which are present in the same ratio by the mass. Consider, for example, that the pure water would always consist of the hydrogen and oxygen atoms in a mass ratio that is fixed. One gram of water approximately contains 0.11 g hydrogen and 0.88 g oxygen in the ratio 1:8.


The law of definite proportions is also called Proust's Law. Below is an image to explain the law of constant proportion. It describes the mass ratio of the elements in some compounds. The ratio of the atoms contained in an element is given below as well. Consider, for example, that in the NO2molecules, the number of nitrogen atoms and the number of oxygen atoms is in the ratio 1:2, however, their mass ratio is found to be 7:16.

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In 1794, Joseph Proust, a French chemist formulated this law of constant proportions from the work he did on sulfates, metal oxides and sulphides. Also, this law was favoured since Dalton’s atomic theory was introduced as well. The relation between them was also discovered by Jacob Berzelius, a Swedish chemist in the year 1811.

Now that we know how to define the law of definite proportions, let us take a look at its exceptions.


Exceptions to the Law of Constant Proportion

The law of constant proportion is a building block for the development of chemistry, however, it does not stand true for all the chemical compounds and has some exceptions. Given below are some of the exceptions to this law.

  1. Some of the non-stoichiometric compounds tend to have varying compositions of the elements between different samples. Hence they tend to follow the law of multiple proportions. 

  2. An example of this is the oxide of iron called wustite, having the formula FeO. The iron and oxygen atoms are in the ratio that ranges from 0.83:1 to 0.95:1. This is because of the crystallographic vacancies present in the samples because of the disordered arrangement of the atoms present.

  3. Different samples of a compound tend to vary in its isotopic composition of the constituent elements. This tends to cause mass ratio fluctuations.

  4. The difference in these mass ratios between the samples are helpful in the geochemical dating because of the preferential concentration of the isotopes in several crustal and deep Earth processes.

  5. This also happens in several atmospheric, astronomical, and oceanic processes. Even if the effects are quite small the challenges faced for the measurement of these effects are overcome by modernly developed instruments.

  6. Since these natural polymers tend to vary in their composition, several samples possess varying mass proportions.

Example of Law of Constant Proportion

  1. Water has both hydrogen and oxygen atoms. One atom of oxygen is combined with two atoms of hydrogen to create the water molecule.

  2. Salt, or NaCl, is composed of Na and Cl atoms. For it to be created, both the sodium and the chlorine atoms have to be in the exact same proportion.

  3. Sulphuric acid has hydrogen, oxygen and sulphur atoms. All these three atoms have to be in the same proportion for the acid to be created.

FAQ (Frequently Asked Questions)

1. State and Explain the Law of Definite Proportion.

Ans: Let us state the law of constant proportion. 

The law of constant proportion is also referred to as Proust’s law or the law of definite proportion. This law states that any given chemical compound consists of its component elements in a fixed mass ratio. It is not dependent on its method of preparation and its source. Hence, if a compound is made when ¼ of one chemical is combined with the ¾ of the other chemical, these proportions would always hold true irrespective of the amount of the chemicals that are added.

This law of constant proportion makes sure that the chemical compounds are created in a way that they have the same proportions always, irrespective of the amount of the compound that is prepared.

2. What is the Law of Constant Composition?

Ans: To understand the law of constant composition, it is essential to state the law of constant proportion. Let us define the law of constant proportion.

The law of constant composition states that in any given chemical compound the elements present in it will always combine in the exact same proportions with one another. 

Now let us state the law of definite proportion in detail with an example.

According to the law of constant composition, all the samples present in the given compound would have the exact same elements in the same mass ratio. Consider, for example, that the water molecules will always have the hydrogen and the oxygen atoms in the ratio 2:1. When we see the relative masses of the oxygen and hydrogen atoms in the water molecule, it is found out that oxygen atoms occupy 94% of the mass and the remaining 6% mass is occupied by the hydrogen atoms. This proportion of the masses remains constant for all the water molecules.

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