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Molecular Formulas

Chemical Substances

There are several ways to classify chemical substances. One scheme uses two major categories: molecular and ionic. Molecular substances are composed of molecules all of which are alike. A molecule is an aggregate of at least two atoms in a fixed spatial arrangement held together by chemical forces (chemical bonds).

Molecular Formulas

A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance. The number of each atoms is shown by means of a subscript. The formula of water for example is H2O. This tells us that a molecule of water, the smallest unit, contains two atoms of hydrogen and one atom of oxygen. When a subscript is one it is omitted from the formula.

Empirical Formulas

Empirical formulas are the simplest chemical formulas, they are written by reducing the subscripts in molecular formulas to the smallest possible whole numbers. Molecular formulas give the actual number of atoms of each element present in a molecule, whereas empirical formulas give the simplest whole number ratio of the atoms in a molecule. For example the molecular formula of benzene is C6H6 and its empirical formula is CH. As we will see later when chemist analyze an unknown compound, the first step is usually the determination of the empirical formula. For many substances the molecular and empirical formulas are the same. Examples are water (H2O), ammonia (NH3) and sulfur dioxide (SO2).

 

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