Chemistry: Empirical and Molecular Formulas

1. DEFINITIONS

Molecular Formula: The molecular formula tells you the actual number of atoms of each element present in a single molecule of a compound. It is the true identity of the substance.

Empirical Formula: The empirical formula is the simplest whole-number ratio of the atoms in a compound. It doesn’t tell you exactly how many atoms are there, just the proportion between them.

Analogy: Think of a carton of eggs. The molecular formula is 12 eggs (the actual count). The empirical formula is 1 dozen (the simplified unit).

2. EXAMPLES

Water: Molecular H2O / Empirical H2O (2:1 cannot be simplified further).

Glucose: Molecular C6H12O6 / Empirical CH2O (All subscripts are divisible by 6).

Hydrogen Peroxide: Molecular H2O2 / Empirical HO (Both subscripts are divisible by 2).

Hydrazine: Molecular N2H4 / Empirical NH2 (Simplified by a factor of 2).

3. HOW TO CALCULATE THE EMPIRICAL FORMULA

If you are given the percent composition, follow these four steps (Rhyme: "Percent to mass, mass to mole, divide by small, multiply 'til whole!"):

Step 1: Percent to Mass - Assume you have a 100g sample. This turns percentages directly into grams (40% becomes 40g).

Step 2: Mass to Mole - Divide the grams of each element by its molar mass from the Periodic Table.

Step 3: Divide by Small - Look at your mole results and divide all of them by whichever number is the smallest.

Step 4: Multiply 'til Whole - If you get a decimal like 1.5, multiply all numbers by a factor (like 2) to get whole numbers.

4. CONVERTING EMPIRICAL TO MOLECULAR

To find the molecular formula, you need the molar mass of the actual compound (usually provided in the problem).

The Scaling Factor (n) = Molar Mass of Compound / Mass of Empirical Formula.

Once you find the value of n, multiply all the subscripts in your empirical formula by n.

Example: If your empirical formula is CH2 (mass of 14) and the problem says the real molar mass is 42, then n = 42/14 = 3. Your molecular formula is C3H6.