Percentage Composition of Comp., Emperical & Molecular Form.

• The formula weight or the molecular weight of a compound represent the total weight of that compound. We can calculate what fraction of the total weight each element in that compound represents. If this fractional weight of each element is expressed as a percentage, we have the percentage composition of a compound.
• Given the formula of a compound, you can calculate the weight percentage of each element. The reverse can also be done: Given the weight percentage of each element in a compound, you can calculate the empirical formula of the compound. The empirical formula gives the lowest ratio of the atoms that are present in the compound.
• In this step the grams are changed to moles. Use the conversion factor.

  number of moles = given weight in grams/weight of one mole of an atom
• The molecular formula, unlike the empirical formula, does not always have the lowest ratio for the subscripts.
• If the molecular formula and the empirical formula are different, the subscripts of the molecular formula will be whole number multiple of the subscripts of the empirical formula.