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By now you should know that a molecular
formula is a group of elemental symbols, and possibly subscript numbers, which
represent the composition of a molecule. The molecular formula shows you how many of
each atom can be found in a certain molecule. Some examples of molecular formulas,
that you are most likely familiar with, are shown in table 5-5a.
Table 5-5a -
Some Molecular Formulas and the Information they Convey |
Formula |
Composition |
H2O |
Total of 3 atoms in each molecule - 1 oxygen and 2 hydrogen |
O2 |
Total of 2 atoms in each molecule - 2 oxygen atoms |
CO2 |
Total of 3 atoms in each molecule - 1 carbon and 2 oxygen |
C6H12O6 |
Total of 24 atoms in each molecule - 6 carbon, 12 hydrogen, and 6 oxygen. |
While the molecular formula shows the
molecule as it actually exists, the empirical formula for a compound
shows the simplest whole number ratio for the elements in the compounds. To find the
empirical formula for a compound, find the largest whole number that can be divided into
each of the subscripts evenly. If no number will go into any of the subscripts
evenly, then the empirical formula and the molecular formula for the compound are the
same. Table 5-5b shows examples of some empirical and molecular formulas.
Table 5-5b -
Comparing Empirical and Molecular Formulas |
Compound |
Molecular Formula |
Empirical Formula |
| Water |
H2O |
H2O |
| Hydrogen Peroxide |
H2O2 |
HO |
| Glucose |
C6H12O6 |
CH2O |
| Methane |
CH4 |
CH4 |
| Ethane |
C2H6 |
CH3 |
| Octane |
C8H18 |
C4H9 |
Molecular and
Empirical Formulas Quizzes |
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Please forward all questions, comments and criticisms to Gregory L. Curran.
© Copyright 2004 Fordham Preparatory School, All Rights Reserved.
Last Modified February 07, 2008 |
