Empirical Formula
The simplest whole number ratio of atoms in a compound.
e.g., CH₂O (Glucose's empirical formula).
Molecular Formula
The actual number of atoms of each element in one molecule.
e.g., C₆H₁₂O₆ (Glucose's molecular formula).
Key Relationship
\( \text{Molecular Formula} = (\text{Empirical Formula}) \times n \)
where \(n = \frac{M_r \text{ (molecular)}}{M_r \text{ (empirical)}}\)
Determining Empirical Formula from Mass Data
Problem: A compound contains 40.0% Carbon, 6.7% Hydrogen, and 53.3% Oxygen by mass. Find its empirical formula.
| Element | Mass (assume 100g) | Moles | Divide by smallest | Ratio |
|---|---|---|---|---|
| C | 40.0 g | \(\frac{40}{12} = 3.33\) | \(\frac{3.33}{3.33}\) | 1 |
| H | 6.7 g | \(\frac{6.7}{1.01} = 6.63\) | \(\frac{6.63}{3.33}\) | 2 |
| O | 53.3 g | \(\frac{53.3}{16} = 3.33\) | \(\frac{3.33}{3.33}\) | 1 |
Answer: CH₂O