PERCENTAGE COMPOSITION AND MOLECULAR FORMULA



For the systematic study of a compound that first step is to determine its chemical formula. This can be achieved by analyzing the compounds for the amounts of the elements (moles) in a given mass of the compound. The results so obtained are expressed as the percentage composition – that is, as the mass percentage of each element in the compound.



    According to law of definite proportions any sample of a pure compound always consists of the same elements combined in the same proportions by mass. Every molecule of ammonia always has the formula NH3. i.e. 1 mol of ammonia always contains 1 mol of N and 3 mol of H. In other words, 17.0 g of NH3 always contains 14.0 g of N and 3.0 g of H.



    The molecular composition can be expressed in either of the following three equivalent ways:



(i)    A formula giving the number of atoms of each type per molecule



(ii)    Number of moles of each element per mole of a compound



(iii)    Mass of each element per 100g of a compound (per cent composition)