3.5 Determining Empirical and Molecular Formulas

Remember that the empirical formula of a compound tells you the simplest ratio of elements in the compound. On the other hand, the molecular formula tells you the actual ratio of elements in the compound.

Determining the empirical formula of a compound involves the following steps: Step 1. Find the number of moles of each element from its mass or percentage composition. Step 2. Divide each element's molar amount by the smallest or least molar amount to get the simplest whole- number ratio of the compound. Step 3. Write the empirical formula using the simplest ratio as subscripts. Determining the molecular formula of a compound involves the following steps: Step 1. Find the molar mass of the empirical formula. Step 2. Divide the compound's molar mass by the empirical formula's molar mass. This will give you a whole number (n). Step 3. Multiply all the subscripts in the empirical formula by n to get the molecular formula.

Example 3.6

A compound has the following composition:

Carbon $= 40\%$ , hydrogen $= 6.7\%$ and oxygen $= 53.3\%$

The molar mass of the compound is 180 g/mol. Determine its empirical and molecular formulas.

Solution

Moles of elements

$$\mathrm{C}=40\mathrm{/}12=3.33$$$\mathrm{C} = 40 / 12 = 3.33$

$$\mathrm{H}=6.7\mathrm{/}1=6.7$$$\mathrm{H} = 6.7 / 1 = 6.7$

$$\mathrm{O}=53.3\mathrm{/}16=3.33$$$\mathrm{O} = 53.3 / 16 = 3.33$

Mole ratio

$$\mathrm{C}:\mathrm{H}:\mathrm{O}$$$\mathrm{C}:\mathrm{H}:\mathrm{O}$