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Empirical and Molecular Formula
The simplest whole number ratio in which the atoms in a compound exist.
The number which shows the number of each type of atom in one molecule of a particular
substance. It is always a simple whole number multiple of the empirical formula.
A substance contains 85.8% carbon and 14.2% hydrogen, what is its empirical formula? If its
relative molecular mass is 56, what is its molecular formula?
Mole Ratio = C: 85.8/12 H: 14.2/1
= 7.15 so 7.15/7.15 = 14.2 so 14.2/7.15
Empirical Formula = CH2
RMM = 56 = (CH2) so 14n = 56 and n = 56/14 = 4
Molecular formula = C4H8
t is also possible to calculate the percentage composition by mass of a substance, if its
empirical or molecular formula is known.
What is the percentage composition by mass of ethanoic acid, C2H4O2?
RMM = 60
% C = (12 x 2)/60 x 100 = 40.0%
% H = (1 x 4)/60 x 100 = 6.67%
%O = (16 x 2)/60 x 100 = 53.3%