When studying chemical formulas, they may be written in several different ways. The most common way is via a molecular formula. Molecular formulas tell us the number of atoms of each type of element in a molecule or compound. This formula does not necessarily tell us the order in which the atoms are connected, but will provide the total number of each type of atom. Take the molecular formula for sugar C6H12O6. This is the formula for both glucose and galactose. Both sugars have the same molecular formula, but different structures. In section 5.11 we covered calculating formula mass values. The mass of any compound represents the mass of all atoms of the substance in 1 mole of a substance. We will cover more about moles later, but for now it is important to understand that moles are the way we quantify a grouping of atoms for a specific substance. A great example of this is taking a carton of eggs. We measure eggs by the dozen. It is commonly accepted that one dozen eggs is the same as 12 eggs. When calculating the mass of a substance, it is accepted that the mass of each element shown on the periodic table is the mass for one mole (or one grouping of atoms) for that substance. The units used here for mass will be grams. Hence, if you look at any element on the periodic table the mass shown is the number of grams of atoms in one mole of that substance.
Another way in which a formula may be written is called an empirical formula. Empirical formulas do not tell you the number of teach type of atom that is in a substance, but rather only the types of atoms involved in a substance. Often times empirical formulas are found, and then through taking measurements the molecular formula is determined. We will cover the concept of empirical formulas in this unit.
Determining Empirical Formulas
An empirical formula tells us the relative ratios of different atoms in a compound. The ratios hold true on the molar level as well. Thus, H2O is composed of two atoms of hydrogen and 1 atom of oxygen. Likewise, 1.0 mole of H2O is composed of 2.0 moles of hydrogen and 1.0 mole of oxygen. We can also work backwards from molar ratios since if we know the molar amounts of each element in a compound we can determine the empirical formula.
In a procedure called elemental analysis, an unknown compound can be analyzed in the laboratory in order to determine the percentages of each element contained within it. These percentages can be transformed into the mole ratio of the elements, which leads to the empirical formula.
- A process is described for the calculation of the empirical formula for a compound based on the percent composition of that compound.
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