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Chemical Composition - Definition, Chemical and Physical Properties, Law of Constant Composition


Chemical composition is defined as the type, arrangement and ratio of atoms in molecules of chemical substances. Chemical composition becomes different when the chemicals are added or removed from a substance and the ratio of different substances or other chemicals changes.

The chemical composition of a substance determines the physical and chemical properties of the substance. It can be concluded that the composition defines the way atoms are put together, density, color, strength and other properties associated with the substance.

Any substance which has a specific structure and chemical composition is known as a chemical. Chemicals are known to exist in a range of states i.e., they can be either solid or liquid. Their states can be changed and are influenced by certain factors like pressure, temperature etc.

The Chemist Joseph Proust defined the fundamentals of modern chemistry known as the Law of Constant Composition. A chemical substance has a fixed composition. According to Proust’s inference, all samples of a compound have the same composition; i.e., all samples have the same proportion, by mass, of the elements present in the compound. For example, water is essentially a chemical compound made up of two elements hydrogen and oxygen in a definite ratio. The purified water samples from a borewell and a sea will also have the same compositions in the same ratio as any other sources of water.

Atom is the basic element or unit of chemical substances which consist of subatomic particles such as electrons, neutrons and protons.

An element is composed of the same atom. These can’t be fragmented by simple or ordinary processes. Metals, nonmetals and metalloids are the three classifications of elements. Physical properties, its associated chemical properties and stability are a few of the criteria for the classification.

Two or more elements combine to form a chemical compound. For example, water or H2O, N2O and so on are compounds. When two or more atoms or elements combine, a molecule is formed. For example, N2, O2, H2O are molecules.

Hence, it is said that every compound is a molecule but every molecule is not a compound.

Chemical substances have a fixed composition. They also have a chemical name and formulae based on their composition. Chemists follow the International Union of Pure and Applied Chemistry (IUPAC) or the Chemical Abstracts Service (CAS) rules for naming a compound. At certain instances, chemical compounds obtained have mixed impurities. A purification process might be either a physical or a chemical process.

Calculating the chemical composition of a substance is calculated in terms of percentage and it is relatively easy. The percentage defines the contribution of each element in the substance by its mass. The steps to calculate the chemical composition of elements present in the compound is given below as follows:

  • The initial step is to determine the molar mass of all the elements present in the compound in terms of grams per mol.
  • Next step is to determine the molecular mass of the compound as a whole.
  • Divide the molar mass of the compound with the molecular mass of the compound.
  • The number resulting from division results in a value less than 1. Multiply this number with 100 in order to determine the percentage of composition in the compound in the form of %.

For example, butane has a molecular formula of C4H10. Butane’s percent composition can be calculated as:

Mass of H per mol butane: 10 mol H⋅(1.00794 g /1 mol H) = 10.079 g H

Mass of C per mol butane: 4 mol C x (12.011 g C / 1mol C) = 48.044g C

Mass percent H in butane: (10.079g H / 58.123 g butane) x 100=17.3% H

Mass percent C in butane: (48.044g C / 58.123 g butane) x 100=82.7% C


Therefore, the atomic composition of butane is 17.3% hydrogen and 82.7% carbon, and, as expected, these values sum to 100%.





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