Q.Define empirical and molecular formulae and give the relation between them. How would you determine the empirical formula of a compound from the percentage composition of its elements ?
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Start your 14-day free trial to unlock the full solution →Empirical formula = simplest ratio; molecular formula = actual number = n × empirical formula.
Empirical formula: It represents the simplest whole-number ratio of the atoms of different elements present in a compound. For example, the empirical formula of glucose is CH2O.
Molecular formula: It represents the actual number of atoms of each element present in one molecule of the compound. For glucose it is C6H12O6.
Relation between them:
Molecular formula = n × (Empirical formula)
where n = (Molecular mass) / (Empirical formula mass). For glucose, empirical formula mass of CH2O = 30, molecular mass = 180, so n = 180/30 = 6, giving C6H12O6.
Determining empirical formula from percentage composition (steps):
- Write down the percentage of each element (this is the mass of each in 100 g).
- Divide the percentage of each element by its atomic mass to get the relative number of moles (atoms) of each.
- Divide each of these values by the smallest one to get the simplest ratio.
- If the ratio is not whole numbers, multiply all by a suitable integer to make them whole.
- Write the elements with these whole numbers as subscripts — this is the empirical formula. …
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