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molecular formula from empirical formula
The molecular formula of a compound is a multiple
of its empirical formula. The empirical formula is the simplest
formula of a substance, written with the smallest integers. For
example, the molecular formula of benzene
C6H6is equivalent to the empirical formula,
(CH)6. This means that the molecular weight is some
multiple of the empirical formula weight. The empirical formula
weight is obtained by summing the atomic weights from the empirical
formula. For any kind of molecular compound, we can write:
| Molecular weight = n * empirical
formula weight |
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where n is the number of empirical formula units in a
compound. We can get the molecular formula by multiplying the
subscripts of the empirical formula by whatever n is. We can
calculate this from the equation:
| n = (molecular weight)/(empirical
formula weight) |
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Once we determine the empirical formula of a compound, we can
calculate is empirical formula weight. If we have an experimental
determination of its molecular weight, we can calculate n
and then its molecular formula.
molecular formula example:
A hydrocarbon is 84.25% carbon and 15.75% hydrogen
and has a molecular weight of 114. What is its molecular
formula?
| C 84.25/12 = 7.021 |
7.021/7.021 = 1 x 4 = 4 |
| H 15.75/1 = 15.75 |
15.75/7.021 = 2.25 x 4 = 9 |
114 / 57 = 2 |