To determine the molecular formula from the empirical formula, follow a three-step process based on the relationship between the empirical and molecular formula. The key is to find a multiplier that scales the empirical formula to the molecular one.
Steps to Find the Molecular Formula
Here's a detailed breakdown of how to determine the molecular formula from the empirical formula:
| Step | Action | Explanation |
|---|---|---|
| 1 | Calculate the molar mass of the empirical formula. | Add up the atomic masses of all the atoms in the empirical formula as found on the periodic table. This gives the mass of one mole of the empirical formula. |
| 2 | Divide the given molecular molar mass by the molar mass calculated for the empirical formula. | The result of this division provides the whole-number multiplier to scale up the empirical formula to the molecular formula. The provided molecular molar mass must be specified. |
| 3 | Multiply each subscript in the empirical formula by the whole number obtained from step 2. | This process will give you the subscript values for the molecular formula. |
Example Scenario
Let’s consider an example. Suppose a compound has an empirical formula of CH2O and a molecular mass of 180 g/mol.
-
Calculate the molar mass of the empirical formula (CH2O):
- C: 1 x 12.01 g/mol = 12.01 g/mol
- H: 2 x 1.01 g/mol = 2.02 g/mol
- O: 1 x 16.00 g/mol = 16.00 g/mol
- Total: 12.01 + 2.02 + 16.00 = 30.03 g/mol
-
Divide the given molecular molar mass (180 g/mol) by the molar mass of the empirical formula (30.03 g/mol):
- 180 g/mol / 30.03 g/mol ≈ 6
-
Multiply the subscripts in the empirical formula (CH2O) by the multiplier (6):
- C1x6H2x6O1x6 = C6H12O6
Therefore, the molecular formula of the compound is C6H12O6, which is the formula for glucose.
Key Points
- The empirical formula shows the simplest whole-number ratio of atoms in a compound.
- The molecular formula shows the actual number of atoms of each element in a molecule.
- The molecular formula is a whole-number multiple of the empirical formula.
By following these steps, you can accurately determine the molecular formula from an empirical formula and a given molecular mass.
