Quantitative Analysis of Compounds

Definitions

• Empirical Formula: Represents the relative quantity of each element as the smallest whole-number ratio of atoms.

• Percent Composition: Indicates the percentage by mass of each element in a compound.

Percent Composition

• Can be calculated from a chemical formula or laboratory data.

• Example Calculation for CaCl₂:

  • Molar mass of CaCl₂ = (1 × 40.078 g/mol) + (2 × 35.45 g/mol).

  • Percent of Ca in CaCl₂: ( \frac{\text{grams Ca}}{\text{grams CaCl₂}} \times 100 \% )

Empirical Formulas

• Describes the ratio of elements in a compound, useful when only elemental data is known.

• Example: Empirical formula of glucose is CH₂O; molecular formula is C₆H₁₂O₆.

Empirical Formula from Lab Data

• Example: For a manganese oxide with 69.6% Mn and 30.4% O:

  • Convert percentages to grams (assume 100 g total).

  • Find moles: Mn and O, then find the whole-number ratio to derive the empirical formula.

Molecular Formulas

• Derived from empirical formulas based on molecular weight.

• Example Calculation: If empirical formula is CH₂O and molecular weight is 180.16 amu,

  • Empirical formula weight = 30.03 g/mol.

  • Ratio = ( \frac{180.16}{30.03} ) gives factor to determine molecular formula.

Careers in Science

• Differentiate roles within chemistry: Analytical vs Physical Chemists.