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objectives
Empirical and Molecular Formulae
Empirical and Molecular Formulae
The empirical formula of a compound is the formula which shows the simplest whole-number ratio in which the atoms in that compound exist.
It can be calculated if the composition by mass of the compound is known.
The molecular formula of a substance is the formula which shows the number of each type of atom in the one molecule of that substance.
It applies only to molecular substances, and can be deduced if the empirical formula and molar mass of the compound are known.
The molecular formula is always a simple whole number multiple of the empirical formula.
Worked Example
Worked Example
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 = 85.8 : 14.2
12 1
= 7.15 : 14.2
7.15 : 7.15
= 1 : 2 so empirical formula = CH2
RMM = 56 = (CH2) so 14n = 56 and n = 56/14 = 4
Molecular formula = C4H8
It is also possible to calculate the percentage composition by mass of a substance, if its empirical or molecular formula is known.
Worked Example 2
Worked Example 2
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%
empirical & Molecular Formula Activity
empirical & Molecular Formula Activity
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