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Composition of a substance
The chemical composition of a pure substance corresponds to the relative amounts of the elements that constitute the substance itself. It can be expressed with a chemical formula, such as an empirical or molecular formula.
For example, the chemical formula for water is H2O: this means that each molecule of water is constituted by 2 atoms of hydrogen (H) and 1 atom of oxygen (O). Thus the chemical composition of water may be interpreted as a 2:1 ratio of hydrogen atoms to oxygen atoms.
Composition of a mixture
The chemical composition of a mixture can be defined as the distribution of the individual substances that constitute the mixture, called "components". In other words, it is equivalent to quantifying the concentration of each component. Because there are different ways to define the concentration of a component, there are also different ways to define the composition of a mixture. It may be expressed as molar fraction, volume fraction, mass fraction, molality, molarity or normality or mixing ratio.
Chemical composition of a mixture can be represented graphically in plots like ternary plot and quaternary plot.