CHEMystery: Chemical Reactions: Empirical and Molecular Formulas

Empirical and Molecular Formulas

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Stoichiometric equivalence of two elements in a formula unit is the ratio (by atoms or moles) in which they occur in the formula. For example, in the formula P₄O₁₀, the ratio of P atoms to O atoms is 4:10. In one mole of P₄O₁₀, there are 4 moles of P and 10 moles of 10.
The molecular, or formula mass of a substance is the sum of all the atomic masses which compose the molecule. The gram molecular mass of a compound is the mass (in grams) which equals the molecular mass. For example, carbon dioxide (CO₂) has a formula mass of 44 (12 + 16 + 16) because the one carbon has a mass of 12 and the two oxygens have a mass of 16 each. Therefore, the gram molecular mass is also 44 grams, because 44 grams contains one mole of carbon dioxide.

Percentage Composition

Using stoichiometry we can determine the percentage of a substance in a compound. Percentage composition is determined with respect to other components in the compound. Percentage by mass is equal to the mass of part divided by the mass of the whole. For example, if something weighed 8.657 grams and its elements were 5.217 g of carbon, 0.9620 g of hydrogen, and 2.478 g of oxygen, the percentage composition would be as follows:

For C:   5.217/8.657 = 60.26% C
For H:  0.9620/8.657 = 11.11% H
For O:   2.478/8.657 = 28.62% O

Empirical Formula

An empirical formula represents the simplest atomic ratio within a compound. When you look at propene (C₃H₆) and octene (C₄H₈) empirically they are identical, because their empirical formula is CH₂. To get back to the original formula you must multiply by an integral value (i.e. for propene multiply by 3; for octene multiply by 4).
If the mass ratio is given the empirical formula can be determined. If you know the mass ratio of hydrogen to carbon is 4:1 in a compound that contains only hydrogen and carbon atoms, it can be deduced from this information that for every 4 moles of the hydrogen there is 1 mole of carbon. The empirical formula is therefore CH₄.

Molecular Formula

To find out the molecular formula of a substance, the substance's actual formula mass needs to be known and compared to the calculated value. For example, styrene has an empirical formula of CH. It's molecular mass is 104. For CH, the molecular mass is 13 (12 + 1). The molecular mass of styrene is 104, so dividing 104 by the computed value of 13 gives us 8. We see that styrene is 8 times larger than CH, so the correct molecular formula is C₈H₈.