CALCULATIONS BASED ON FORMULAE

7.8 - Determination of Empirical Formula

In section 7.7, we learned that percent composition establishes the relative proportions of the elements in a compound in terms of mass. From a given chemical formula, we learned to generate a list of percent compositions of the elements that make up the compound.

In this section, we will carry out the opposite task. We will be given the percent compositions of the elements that make up the compound. From that, we will derive the chemical formula of the compound.

For example:

An analysis of a substance shows that it is made up of carbon, hydrogen, and oxygen. The percentage composition of the elements in the compound are determined to be:

• 39.10 % carbon,
• 8.77 % hydrogen, and
• 52.13 % oxygen.

From the above data we can derive the ratio of carbon, hydrogen and oxygen in the substance. We will put it in the form, C_xH_yO_z, where x, y and z are the simplest whole-number ratio of carbon, hydrogen to oxygen of the substance. This chemical formula is called the empirical formula of a substance.

^* It should be noted that the empirical formula may or may not be the true chemical formula of the substance. We will examine this in greater detail in section 7.9.

Section 8.5
Calculation of Empirical (Simplest) and Molecular Formulas ..p200

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