Empirical Formula

How to Calculate the Empirical Formula

To calculate the empirical formula, follow these steps:

1. Determine the Mass Percentages: If not already given, determine the mass percentage of each element in the compound.

2. Convert Mass Percentages to Moles: Convert the mass percentages to moles by dividing each percentage by the atomic mass of the corresponding element.

3. Find the Simplest Ratio: Divide all the mole values by the smallest number of moles calculated in step 2 to find the simplest whole-number ratio of atoms in the compound.

4. Write the Empirical Formula: Use the ratios from step 3 to write the empirical formula, using the element symbols and subscripts to represent the number of atoms of each element.

Example: Calculating the Empirical Formula of a Compound

Suppose you have a compound that is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Let's calculate its empirical formula.

1. Convert Mass Percentages to Moles:

   • Moles of carbon (C): 40.0 g / 12.01 g/mol≈3.33 mol 
   • Moles of hydrogen (H): 6.7 g / 1.008 g/mol≈6.65 mol
   • Moles of oxygen (O): 53.3 g / 16.00 g/mol≈3.33 mol

2. Find the Simplest Ratio:

   • Divide all mole values by the smallest number of moles (3.33 mol):
     • Carbon: 3.33 mol / 3.33 mol ​= 1
     • Hydrogen: $2$
     • Oxygen: $1$

3. Write the Empirical Formula:

   • The simplest whole-number ratio of C:H:O is 1:2:1, so the empirical formula is CH₂O.

Conclusion

The empirical formula of the compound with 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass is CH₂O.