3.3 | formula weights and elemental compositions of substances 

Understanding Formula Weights and Elemental Composition

Introduction

• Sulfuric acid (H₂SO₄) is widely used in the chemical industry and often dispensed in milliliters or its aqueous solution (H₂SO₄ (aq)).

• This section discusses the connection between chemical equations (which represent individual molecules) and laboratory measurements (in grams or milliliters).

Goals of the Section

• Learn to calculate:

  • The formula weight of a substance from its empirical formula.

  • The molecular weight from its molecular formula.

  • The elemental composition based on mass percentages of the elements.

Formula and Molecular Weights

• The formula weight is the sum of the atomic weights of atoms in the chemical formula.

• Example: For sulfuric acid (H₂SO₄):

  • Formula Weight = Atomic Weight of Sulfur + (4 × Atomic Weight of Oxygen) + (2 × Atomic Weight of Hydrogen)

  • Using the periodic table, H₂SO₄ has a formula weight of approximately 98.1 amu.

Atomic Weights Used in Calculations

• Atomic Weight for Sulfur (S): 32.1 amu

• Atomic Weight for Oxygen (O): 16.0 amu

• Atomic Weight for Hydrogen (H): 1.0 amu

Special Cases in Formula Weights

• When the chemical formula represents a single element (e.g., Sodium (Na)), the formula weight equals the atomic weight (Na = 23.1 amu).

• For ionic compounds (e.g., CaCl₂):

  • The empirical formula is used.

  • Formula weight of CaCl₂:

    • Formula Weight = Atomic Weight of Ca + (2 × Atomic Weight of Cl)

    • CaCl₂ = 40.1 amu + 2 × 35.5 amu = 111.1 amu.

Elemental Compositions of Substances

• The elemental composition helps identify substances, particularly in forensic chemistry (e.g., unknown white powder).

• Measuring elemental composition involves:

  • Comparing the calculated elemental compositions of different substances to identify a match.

• Mass Percentage Formula:

  • % Mass of Element = (Mass of Element in the Compound / Total Formula Weight) × 100%

Example: Mass Percentage Calculation of Sulfur in H₂SO₄

• In H₂SO₄, to find the percentage of sulfur:

  • Step 1: Identify Mass of Sulfur: 32.1 amu (1 sulfur atom per H₂SO₄ molecule).

  • Step 2: Calculate:

    • % S in H₂SO₄ = (32.1 amu / 98.1 amu) × 100% = 32.7%.

Conclusion

• Significance: Almost one-third of the mass of pure H₂SO₄ comes from sulfur, which is crucial for understanding its chemical properties and behavior in reactions.