Molecular Formula of Organic Compounds

1. Definition:

• Molecular formula: A chemical formula that represents the number of atoms of each element present in a molecule of an organic compound.

• Example: The molecular formula of methane is CH₄, indicating that one molecule of methane contains one carbon (C) atom and four hydrogen (H) atoms.

• Empirical formula: This formula represents the simplest whole-number ratio of atoms in a compound.

• Example: The empirical formula of ethylene is CH₂, signifying a 1:2 ratio of carbon to hydrogen atoms.

2. Relationship Between Molecular and Empirical Formulas:

• The molecular formula is an integral multiple of the empirical formula.

• Example: The molecular formula of glucose is C₆H₁₂O₆, while its empirical formula is CH₂O.

• Knowing the empirical formula and the molar mass of a compound, you can determine its molecular formula.

3. Determining the Molecular Formula:

To determine the molecular formula of an organic compound, follow these steps:

• Step 1: Determine the empirical formula by elemental analysis, which provides the percentage composition by mass of each element.

• Step 2: Determine the molar mass (M) of the compound using experimental techniques such as boiling point, freezing point, or vapor pressure measurements.

• Step 3: Calculate the whole-number multiplier for the empirical formula by dividing the molar mass by the empirical formula mass.

4. Important Points:

• The molecular formula doesn’t reveal the structure of the molecule, meaning it doesn’t indicate how atoms are connected.

• Multiple organic compounds can share the same molecular formula but have different structures, leading to varying properties. This phenomenon is known as isomerism.

5. Calculating Molar Mass (M):

• M = MB. dA/B, where:

• M: Molar mass of the substance (g/mol).

• MB: Molar mass of the standard substance (g/mol).

• dA/B: Vapor density of substance A relative to standard substance B (unitless).

Example: The vapor density of methane (CH₄) relative to air is 0.55. Calculate the molar mass of methane.

• The molar mass of air is approximately 29 g/mol.

• Using the formula: M = MB. dA/B = 29 x 0.55 = 15.95 g/mol.

Note:

• Vapor density of substance A relative to standard substance B is the ratio of the mass of a fixed volume of substance A to the mass of the same volume of standard substance B under the same conditions of temperature and pressure.

• When the standard substance B is air, its molar mass is typically taken as 29 g/mol.

Conclusion:

Molecular and empirical formulas serve as crucial tools in organic chemistry, providing insights into the elemental composition and structural makeup of organic compounds.