1. Determine the Empirical Formula
* This is usually provided in the problem, or you may need to calculate it from experimental data (like percent composition).
2. Calculate the Empirical Formula Mass
* Add up the atomic masses of all the atoms in the empirical formula.
3. Determine the Molecular Mass
* This is usually given in the problem. If not, you might need to find it using techniques like mass spectrometry.
4. Find the Whole-Number Ratio
* Divide the molecular mass by the empirical formula mass. This will give you a whole number or a number very close to a whole number.
5. Multiply the Empirical Formula by the Whole-Number Ratio
* Multiply the subscripts in the empirical formula by the whole number you just calculated. This gives you the molecular formula.
Example:
Let's say we have an empirical formula of CH2O and a molecular mass of 180 g/mol.
1. Empirical Formula: CH2O
2. Empirical Formula Mass: 12.01 (C) + 2(1.01) (H) + 16.00 (O) = 30.03 g/mol
3. Molecular Mass: 180 g/mol
4. Whole-Number Ratio: 180 g/mol / 30.03 g/mol ≈ 6
5. Molecular Formula: (CH2O) * 6 = C6H12O6 (glucose)
Key Points
* Empirical Formula: The simplest whole-number ratio of atoms in a compound.
* Molecular Formula: The actual number of atoms of each element in a molecule.
* Molecular Mass: The mass of one molecule of the compound.
* The whole-number ratio tells you how many times the empirical formula is repeated in the molecular formula.
Let me know if you'd like to try an example together!
