Thursday, May 26, 2011

Law of Constant Proportion / Definite Proportion / Fixed Proportion

Louis Proust in 1799 after analysis of large number of compounds arrived at the generalized statement of the law of constant proportion. It states that, “A pure chemical compound, irrespective of its source of method of preparation, always contains the same elements combined together in same definite proportions by weight.”
Thus, if the element ‘A’ and ‘B’ combine chemically to form the compound AB, then in whatever manner AB is formed, it is always composed of same two elements ‘A’ and ‘B’ combined together in the same fixed ratio or proportion by weight.

For example: Sulphur dioxide can be obtained when
         (i).     Sulphur is burnt in air,    
                                    S   +    O2------>SO2


         (ii).    Copper is heated with conc. sulphuric acid

                                    Cu  +   2H2SO4 ------------>  CuSO4   +   2H2O   +   SO2

         (iii).   Dilute hydrochloric acid is added to sodium bisulphate

                                    NaHSO3   +   HCl  ---------->   NaCl   +   H2O   +   SO2

In each case, sulphur and oxygen in the sulphur dioxide obtained are found to be in the same ratio of 32 : 32 or 1 : 1 by weight.

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