Theory
Atoms of different elements chemically combine to form different compounds following
two fundamental laws. The Law of constant composition states that a simple of a pure
compound is always composed of the same element combined in the same proportion by mass.
If we analyze sample of water, we will find that the ratio of the masses of hydrogen and oxygen
in each sample is the same, regardless of the source of the water. The law of the multiple
proportions states that if two elements combine to form a series of compounds, the mass ratio of
the element are small whole numbers. For example, hydrogen forms two compounds with
oxygen: water (H2O) and hydrogen peroxide (H2O2
). In the first compound, 16.0 grams of
oxygen (one mole of O) combine with 2.016 grams of hydrogen (two moles of H); and in the
second compound, 32.0 grams of oxygen (two moles of O) combine with 2.016 grams of
hydrogen (two moles of H). The ratio of masses of oxygen that combine with the hydrogen in
these compounds is 16.0 g/32.0 g or 1:2, which supports the law of multiple proportion.
When a new compound is prepared, its formula must be determined. This task is most
commonly achieved form the percent composition of the compound-the percent by mass of
each element relative to the total mass of the compound. It is obtained by dividing the mass of
each element in the compound by the molar mass of the compound, and multiplying by 100. In
one mole of water (MW = 18.02 g/mol) are two moles of H (2.016 g) and one mole of O (16.0 g).
Therefore, %H = (2.016/18.016)100 = 11.19 percent and %O = (16.0/18.016)100 = 88.81
percent.
The simplest whole-number ratio of the number of atoms of each element in a
compound is called the empirical formula of the compound. The molecular formula shows the
actual number of each type of atom present in a compound. For example, suppose that the
empirical formula CH is obtained experimentally from the percent composition of a compound.
This formula may represent a molecule having the molecular formula form the empirical formula
one must know the molar mass of the compound.
The empirical formula of the compound can be determined either by chemical analysis
or by synthesis. In chemical analysis, a known mass of the compound is decomposed to obtain
the masses of the elements themselves or some of their known derivatives (for carbon, the
derivative is CO2
and for hydrogen, it is water). From these masses, the percent composition of
the compound is determined. In synthesis, known masses of the elements are allowed to form a
compound and the mass of the compound is determined. Form these data, it is possible to
derive the empirical formula of the compound.