Understanding the Concepts
* 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.
Steps to Calculate the Molecular Formula
1. Determine the Empirical Formula Mass: Add the atomic masses of all the atoms in the empirical formula.
2. Determine the Molecular Mass: This is usually provided in the problem. You can also find it experimentally using techniques like mass spectrometry.
3. Calculate the Ratio: Divide the molecular mass by the empirical formula mass.
4. Multiply the Empirical Formula: Multiply the subscripts in the empirical formula by the ratio you calculated in step 3. This gives you the molecular formula.
Example:
Let's say you have a compound with:
* Empirical Formula: CH₂O
* Molecular Mass: 180 g/mol
1. Empirical Formula Mass: (12.01 g/mol C) + (2 * 1.01 g/mol H) + (16.00 g/mol O) = 30.03 g/mol
2. Ratio: 180 g/mol / 30.03 g/mol ≈ 6
3. Molecular Formula: Multiply the subscripts in CH₂O by 6: C₆H₁₂O₆
Therefore, the molecular formula of the compound is C₆H₁₂O₆ (glucose).
Key Points
* The molecular formula will always be a whole-number multiple of the empirical formula.
* If the molecular mass and empirical formula mass are the same, the empirical formula is also the molecular formula.
* You can use the molecular formula to calculate the percentage composition by mass of each element in the compound.
