Empirical and Molecular Formulas in GOB Chemistry

Empirical and Molecular Formulas

Introduction

Understanding the difference between empirical and molecular formulas is essential in GOB Chemistry. These formulas help chemists determine the composition and structure of chemical compounds.

• 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 of the compound.

The molecular formula gives the actual number of different elements in a given compound.

Comparison Table: Empirical vs. Molecular Formulas

Compound	Empirical Formula	n-factor	Molecular Formula
Glucose	CH2O	6	C6H12O6
Octane	C4H9	2	C8H18
Salicylic Acid	C7H6O3	1	C7H6O3

Calculating the Molecular Formula

Step-by-Step Process

Once the empirical formula is determined, the molecular formula can be calculated if the molar mass is known.

1. Step 1: Repeat the steps necessary to determine the empirical formula of the compound.

   • Convert mass percentages to moles.

   • Divide by the smallest number of moles to get the simplest ratio.

2. Step 2: Calculate the empirical mass of the compound.

   • Sum the atomic masses of all atoms in the empirical formula.

3. Step 3: Divide the molar mass of the molecular formula by the empirical mass to determine the n-factor.

   • Formula: $n = \frac{\text{Molar Mass}}{\text{Empirical Mass}}$

4. Step 4: Multiply the subscript of the empirical formula by the n-factor to get the molecular formula.

   • Formula: $\text{Molecular Formula} = n \times (\text{Empirical Formula})$

Example Problem

Example: After a workout session, lactic acid (M = 90.08 g/mol) forms in muscle tissue and is responsible for muscle soreness. Elemental analysis shows that this compound contains 40% C, 6.7% H, and 53.3% O. Determine the molecular formula.

• Convert percentages to moles for each element.

• Find the simplest whole-number ratio (empirical formula).

• Calculate empirical mass.

• Divide molar mass by empirical mass to find n-factor.

• Multiply empirical formula by n-factor to get molecular formula.

Practice Problems

Practice 1

Question: What is the molecular formula for the following compound?

• Empirical Formula: NPC2

• Molar Mass: 347.64 g/mol

Use the steps above to solve.

Practice 2

Question: Cortisol (M = 362.47 g/mol), a known steroid hormone, is found to contain 69.6% carbon, 8.34% hydrogen, and 22.1% oxygen by mass. What is its molecular formula?

Practice 3

Question: Elemental analysis of a pure compound indicated that the compound had 72.2% C, 8.50% H and the remainder as O. If 0.250 moles of the compound weighs 41.55 g, what is the molecular formula of the compound?

Key Terms and Definitions

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

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

• Molar Mass: The mass of one mole of a substance, expressed in grams per mole (g/mol).

• n-factor: The ratio of the molar mass to the empirical mass, used to scale the empirical formula to the molecular formula.

Summary Table: Steps to Determine Molecular Formula

Step	Description	Formula
1	Find empirical formula	Convert % to moles, find simplest ratio
2	Calculate empirical mass	Sum atomic masses
3	Find n-factor	$n = \frac{\text{Molar Mass}}{\text{Empirical Mass}}$
4	Determine molecular formula	$\text{Molecular Formula} = n \times (\text{Empirical Formula})$

Additional info: The notes are focused on the calculation and understanding of empirical and molecular formulas, which are core topics in GOB Chemistry. Practice problems reinforce the calculation steps and application to real compounds.