Law of Definite Proportions: Definition, Examples, and Types

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What is Law Of Definite Proportions?

This law of definite proportions is the basis for the study of stoichiometry in Chemistry.

The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio in terms of their mass.

This specifies that any pure sample of a compound, (no matter what is the source of that compound), will always encompass the same elements that are present in a similar ratio by their mass.

The Law of Definite Proportions also shows that whatever the quantity of water is, whether it be 4 moles or 72 grams, the ratio of the amount of hydrogen to oxygen by its weight will always remain the same, just like that of egg:sugar: butter ratio in the chocolate cake is always constant to maintain the delicious taste of the cake.

Law of Definite Proportions Examples

• For instance, in a nitrogen dioxide (NO2) molecule, the ratio of the number of nitrogen atoms present in given compound and oxygen atoms is 1:2 always. This ratio between the nitrogen and oxygen molecules would always remain the same.

• Salt which is written as the chemical compound NaCl comprises of atoms of Sodium (Na) and Chlorine (Cl). Thus, to create salt the exact same proportions of sodium and chlorine must always be combined.

• Sulfuric acid comprises of the specific elements of hydrogen, sulfur, and oxygen. The chemical formula of sulfuric acid is written as H₂SO₄. Thus, to create sulfuric acid the same proportions of hydrogen, sulfur, and oxygen must be combined.

• Ammonia which is a common household item is made up of the elements of hydrogen and nitrogen. The chemical formula for Ammonia is written as NH3, meaning that there is one atom of nitrogen which is combined with 3 atoms of hydrogen. Anhydrous ammonia consists of 82% nitrogen and 18% hydrogen. Any other grouping of hydrogen and nitrogen would result in a completely different type of chemical compound.

Law of Definite Proportions Applications

The law of definite composition has applications of both the compounds;

• molecular compounds having a fixed composition

• whereas ionic compounds require certain ratios to achieve electrical neutrality

• There are certain exceptions to the law of definite composition. These compounds are thus known as non-stoichiometric compounds, and examples of the same include ferrous oxide

• Further, this law of definite composition does not justify for isotopic mixtures.

Law of Definite Proportions and Non-Stoichiometric Compounds / Isotopes

There exist certain non-stoichiometric compounds whose elemental composition can vary from one to another. Such kinds of compounds obey the Law of multiple proportions.

For example, the iron oxide, which may hold iron atoms between 0.83 and 0.95 for each of the oxygen atoms, and consequently comprise anywhere between a percentage of 23 and 25 oxygen by mass.

The ideal chemical formula is given as FeO, but because of the crystallographic vacancies, it is up to FeO.95O.

Basically, Proust’s measurements were not precise enough to spot such small differences. The element’s isotopic composition can also differ based on its source; therefore, its involvement in even a pure stoichiometric compound mass can vary.

This discrepancy can be used in radiometric dating since atmospheric, astronomical, crustal, oceanic, and deep Earth processes can concentrate limited environmental isotopes favorably.

With the hydrogen and its isotope exception, generally, the effect is slight, but it is assessable with modern-day instrumentation.

Law of Definite Proportions Citations


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