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4.6 Empirical Formulas
The empirical formula is a formula that gives the simplest whole
number ratio of atoms in a compound.
For example, C
2
H
4
, C
3
H
6
, and C
4
H
8
all contain twice as many H
atoms than C atoms. Therefore, the empirical formula of all these
molecules is CH
2
.
The simplest ratio of C and H atoms is 1C : 2H
To find the empirical formula for chemistry problems, we will be
doing the opposite of finding the percent composition of a compound.
We can follow these steps:
1.
Find the number of moles present of each element in the
compound.
2.
Divide each of the number of moles by the
smallest
number of
moles calculated.
3.
Find the simplest
whole
number ratio from these numbers.
Ex) A 25.00 g sample of an orange compound is found to contain
6.64 g of Potassium, 8.84 g of Chromium, and 9.52 g of Oxygen.
Find the empirical formula.
If you are given percentages instead of masses, a good strategy is to
assume there is a total of 100 g of the substance.
Ex) A compound consists of 72.2% magnesium and 27.8% nitrogen
by mass. What is the empirical formula?
