The empirical formula is the simplest formula of a compound which gives the simplest whole-number ratio of atoms of the combining elements. Example is the formula of the compound benzene. The molecular formula of benzene is C6H6. This can be written in its simplest form as CH. Thus CH is the empirical formula of benzene.
Molecular formula of a compound shows the actual or effective number of atoms present in the compound. For magnesium chloride (MgCl2), its molecular formula is MgCL2 which indicates it has exactly two atoms of chlorine for every atom of magnesium. The simplest whole number ratio of atoms is 1:2, that is, the empirical formula is also MgCL2.
Chemical equations are shorthand notations used by chemists to describe chemical changes resulting from the regrouping of atoms, molecules or ions to form other substances. Symbols and formulae are used for describing all substances and their compositions involved in the equations of the reaction. Numbers placed before any symbols or formulae in the equations are called coefficients and are used to indicate the simplest whole-number ratios of the atoms, molecules or ions. An equation comprises the reactants which are the substances to the left of the arrow indicating the direction of the reaction and the products usually placed to the right of the arrow. For example, the reaction of sodium and water is represented as:
This means that two moles of sodium react with two moles of water to produce two moles of sodium hydroxide and one mole of hydrogen.
Also the decomposition of water can be represented as shown:
But the equation is not balanced since it does not contain the same number of atoms of each element on both sides of the arrow. We can therefore balance the equation by writing the proper coefficients on appropriate reactants and products on each side of the arrow.
This procedure is referred to as balancing the equation.
Most simple chemical equations can be balanced by inspection, that is by trial and error process in which several sets of coefficients are tried until the correct ones are found. This involves looking at the equation and adjusting the coefficients so that equal numbers of atoms of each type are present on both sides of the arrow. Another example is the reaction of aluminium with hydrogen chloride to produce aluminium chloride and hydrogen.
Chemical equations, in addition to identifying reactions and products, often give other information such as the physical state of the reactants and products. Consider the equation:
In this equation the (g) indicates a gaseous hydrogen, (I) indicates liquid, (s) indicates a solid while (aq) indicates a substance dissolved in water (aqueous).
Special reaction conditions such as temperature or any other special circumstances that characterise the reaction may be written above or below the arrow, e.g. the electrolysis of water is sometimes written as: