Types of Formulas and Interconversion of Representations
- Consider that you're in a chemistry lab, analyzing a small vial of an unknown organic compound.
- You’re tasked with figuring out its molecular structure.
- Where do you begin?
- You might start by examining its formula, but which one?
- A single compound can be represented in multiple ways: as an empirical formula, molecular formula, structural formula, or skeletal formula.
Empirical Formula: The Simplest Ratio
Definition
Empirical formula
The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.
It tells you the relative proportions of atoms but not the actual number of atoms in a molecule.Example
- Glucose has the molecular formula C₆H₁₂O₆, but its empirical formula is CH₂O.
- This means that, for every carbon atom, there are two hydrogens and one oxygen in the simplest ratio.
Key Features:
- Provides no structural information.
- Useful for understanding the relative composition of a compound.
- If a compound has 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass, its empirical formula can be determined.
- Divide each percentage by the atomic mass of the element:
- Carbon:$( \frac{40}{12.01} = 3.33 $
- Hydrogen: $ \frac{6.7}{1.01} = 6.63 $
- Oxygen: $ \frac{53.3}{16.00} = 3.33 $
- Divide by the smallest value (3.33) to get the ratio: C₁H₂O₁, or simply CH₂O.
- When calculating empirical formulas, ensure all percentages add up to 100%.
- If not, account for rounding errors or missing data.
Molecular Formula: The Actual Atom Count
Definition
Molecular formula
The molecular formula specifies the exact number of atoms of each element in a molecule.
For glucose, the molecular formula C₆H₁₂O₆ shows that it contains six carbons, twelve hydrogens, and six oxygens.
Key Features:
- Reflects the actual composition of a molecule.
- Can be derived from the empirical formula if the molar mass is known.
- To find the molecular formula from the empirical formula, divide the molar mass of the compound by the molar mass of the empirical formula.
- Multiply the subscripts in the empirical formula by this factor.
- Students sometimes confuse the molecular formula with the structural formula.
- Remember, the molecular formula does not show how atoms are connected.
Structural Formula: Connectivity of Atoms
Definition
Structural formula
The structural formula shows how atoms are connected in a molecule.
There are three main types:
