Percent Composition, Empirical Formulas and Molecular Formulas

Percent Composition

• Definition: The percentage by mass of each element in a compound.

• Formula:

  • Percent Composition = (Part / Whole) * 100%

Empirical Formulas

• An empirical formula represents the simplest whole number ratio of elements in a compound.

Molecular Formulas

• A molecular formula indicates the actual number of atoms of each element in a molecule of a compound, which can be a multiple of the empirical formula.

Example: Percent Composition of Potassium Permanganate (KMnO4)

• Molar Mass Calculation:

  • Potassium (K): 1 × 39.1 g = 39.1 g

  • Manganese (Mn): 1 × 54.9 g = 54.9 g

  • Oxygen (O): 4 × 16.0 g = 64.0 g

• Total Molar Mass of KMnO4 = 39.1 g + 54.9 g + 64.0 g = 158 g

Calculating Percent Composition:

• % K:

  • (39.1 g / 158 g) × 100 = 24.7%

• % Mn:

  • (54.9 g / 158 g) × 100 = 34.8%

• % O:

  • (64.0 g / 158 g) × 100 = 40.5%

Example: Percent Composition of Sodium Carbonate (Na2CO3)

• Molar Mass Calculation:

  • Sodium (Na): 2 × 23.0 g = 46.0 g

  • Carbon (C): 1 × 12.01 g = 12.01 g

  • Oxygen (O): 3 × 16.0 g = 48.0 g

• Total Molar Mass of Na2CO3 = 46.0 g + 12.01 g + 48.0 g = 106 g

Calculating Percent Composition:

• % Na:

  • (46.0 g / 106 g) × 100 = 43.4%

• % C:

  • (12.0 g / 106 g) × 100 = 11.3%

• % O:

  • (48.0 g / 106 g) × 100 = 45.3%

Example: Percent Composition of Ethanol (C2H5OH)

• Percent Composition:

  • % C = 52.13%

  • % H = 13.15%

  • % O = 34.72%