Molecular Formula: Calculation and Practice in Chemistry

Concept: Molecular Formula

Definition and Importance

The molecular formula of a compound provides the actual number of atoms of each element present in a molecule. It is essential for understanding the composition and properties of chemical substances.

• Empirical Formula: The simplest whole-number ratio of elements in a compound.

• Molecular Formula: The actual number of atoms of each element in a molecule.

• n-factor: The ratio between the molecular and empirical formulas.

Comparison Table: Empirical vs. Molecular Formula

Compound	Empirical Formula	n-factor	Molecular Formula
Octane	CH2	8	C8H18
Glucose	CH2O	6	C6H12O6
Salicylic Acid	C7H6O2	1	C7H6O2

Calculating the Molecular Formula

Step-by-Step Process

To determine the molecular formula, follow these steps:

1. Step 1: Find the empirical formula mass. Example: For C2H6O, empirical formula mass = (2 × 12.01) + (6 × 1.008) + (1 × 16.00) = 46.068 g/mol.

2. Step 2: Calculate the molar mass of the compound (usually given or determined experimentally).

3. Step 3: Divide the molar mass by the empirical formula mass to determine the n-factor: $n = \frac{\text{Molar Mass}}{\text{Empirical Formula Mass}}$

4. Step 4: Multiply the subscripts in the empirical formula by the n-factor to get the molecular formula. $\text{Molecular Formula} = \text{Empirical Formula} \times n$

Example Calculation

Example: Lactic acid (C3H6O3) has a molar mass of 90.08 g/mol. Its empirical formula is CH2O (empirical mass = 30.03 g/mol). $n = \frac{90.08}{30.03} = 3$ Molecular formula: (CH2O)3 = C3H6O3

Practice Problems

Problem 1

Given: Empirical Formula: NPC Molar Mass: 347.64 g/mol Find: Molecular formula.

Problem 2

Given: Cortisol (C21H30O5, molar mass = 362.47 g/mol) contains 69.05% carbon, 8.34% hydrogen, and 22.71% oxygen by mass. Find: Molecular formula.

Problem 3

Given: A compound contains 72.2% C, 8.50% H, and the remainder is O. If 0.250 moles of the compound weighs 41.5 g, what is the molecular formula?

Key Terms

• Empirical Formula: Simplest ratio of elements in a compound.

• Molecular Formula: Actual number of atoms of each element in a molecule.

• Molar Mass: Mass of one mole of a substance, measured in g/mol.

• n-factor: Ratio of molar mass to empirical formula mass.

Summary

Understanding how to determine the molecular formula from the empirical formula and molar mass is a fundamental skill in chemistry. It allows chemists to deduce the actual composition of compounds and is essential for further study in chemical reactions and stoichiometry.