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132                                                                                     ICSE Chemistry – 10

                                           How is the percentage composition of a compound
                                           determined from its formula
                                           The percentage composition of a compound means the mass percentage
                                           of each element present in the compound. The mass percentage of each
                                           element can be determined from the molecular or empirical formula of the
                                           compound as follows.
                                           Step 1.  Write the molecular or empirical formula of the compound.
                                           Step 2.  Write the name, number of atoms, and the total mass of each element
                                                  present in one molecule (or formula) of the compound.
                                           Step 3.  Obtain the molecular mass or formula mass of the compound by adding
                                                  the masses of all the atoms present in it.
                                           Step 4.  Calculate the mass percentage of each element by using the relationship:
                                              Total mass of the element in one molecule of the compound
               Mass percentage of an element =                                                          × 100
                                                          Molecular mass of the compound
                                                This method of calculating the percentage composition of a compound
                                           from its molecular formula is illustrated through the following examples:

                                           EXAMPLE 5.17.  Calculate the mass percentage of each element present in
                                           water (H O).
                                                   2
                                           Solution: The molecular formula of water is H O.
                                                                                    2
                                           A molecule of water is made up of two hydrogen atoms and one oxygen atom.
                                           Thus, one can write,
                                           Molecular formula of water:              H O  = 2 atoms of H + 1 atom of O
                                                                                     2
                                           We know                      Atomic mass of H  = 1 u
                                           and                          Atomic mass of O  = 16 u
                                           Therefore,                Mass of two H atoms  = 2 × 1 = 2 u
                                                                      Mass of one O atom  = 1 × 16 = 16 u
                                           and                    Molecular mass of water  = 2 u + 16 u = 18 u
                                           So,
                                                                                           2 u
                                                  Mass percentage of hydrogen (H) in water  =    × 100 = 11.11
                                                                                           18 u
                                                                                           16 u
                                           and       Mass percentage of oxygen (O) in water  =    × 100 = 88.89
                                                                                           18 u
                                           Therefore, water (H O) contains 11.11% of hydrogen (H), and 88.89% of oxygen
                                                             2
                                           (O).
                                           EXAMPLE 5.18.  Calculate the mass percentage of oxygen in (a) HgO
                                           (b) K Cr O  (c) Al (SO ) . The atomic masses (in u units) are:
                                                     7
                                                           2
                                               2
                                                  2
                                                               4 3
                                                         Hg = 200.6, O = 16, K = 39, Cr = 52, Al = 27, S = 32
                                           Solution: (a)                HgO  { 1 atom of Hg + 1 atom of O
                                           So,         Molecular mass of HgO  = (200.6 u + 16 u) = 216.6 u
                                           Therefore,
                                                       Percentage of O in HgO  =   Mass of oxygen  × 100
                                                                                 Mass of HgO
                                                                                 16 u
                                                                             =         × 100 = 7.39
                                                                                216.6 u
                                           Thus, HgO contains 7.39% oxygen.
                                                    (b)   K Cr O   {  2 atoms of K   +  2 atoms of Cr  +  7 atoms of O
                                                              2
                                                                 7
                                                           2
                                                                        2 × 39 u          2 × 52 u        7 × 16 u
                                                                          78 u             104 u           112 u
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