Chapter 6 Part C: Chemical Composition

Learning Objectives

• Discuss mass percent composition
  • Definition
  • Calculations
• Discuss molecular formula vs empirical formula
  • Definitions
  • Examples
  • Differences
  • Calculating empirical formula from experimental data
  • Determining molecular formula from empirical formula and molar mass

Mass Percent Composition

• Definition: The percent of the total mass of a compound that is due to a specific element.
  • Formula: (Mass of element X / Total mass of compound) x 100%

Example Calculation

• Given: 1.91 grams of sample decomposed into 0.69 grams of calcium and 1.22 grams of chlorine.
• Mass Percent of Calcium: 0.69 / 1.91 = 36.09%
• Mass Percent of Chlorine: 1.22 / 1.91 = 63.91%
• The sum should be 100%, a way to check calculations.

From Chemical Formula

• Formula: (Mass of element X in 1 mole of compound / Molar mass of compound) x 100%
• Example: Calculate mass percent of iron and chlorine in iron(III) chloride.
  • Molar mass of iron(III) chloride = 162.20 g/mol
  • Iron: 34.43%
  • Chlorine: 65.54%

Empirical vs Molecular Formula

• Molecular Formula: Actual number of each type of atom in a molecule (e.g., C₂H₆).
• Empirical Formula: Simplest whole-number ratio of atoms in a compound (e.g., CH₃).
• Examples demonstrate the use of empirical formulas to represent a family of compounds.

Determining Empirical Formula

1. Convert percent composition to grams (assume 100g sample).
2. Convert grams to moles.
3. Determine ratio by dividing by the smallest number of moles.
4. Adjust to whole numbers if necessary.

Determining Molecular Formula

• Use the ratio of the molar mass to the empirical formula mass to find the multiplier (n).
• Formula: Molecular formula = Empirical formula × n

Ionic vs Covalent Compounds

• Ionic Compounds: Consist of repeating units with no distinct molecule (e.g., NaCl).
• Covalent Compounds: Consist of distinct molecules (e.g., H₂O).

Determining Formula for Ionic Compounds

• Only empirical formula is determined due to repeating nature.
• Example: Iron and chlorine composition given, determine empirical formula.

Summary of Chapter 6 Part C

• Discussed mass percent composition with examples and calculation methods.
• Explored differences between molecular and empirical formulas.
• Calculated empirical formula from experimental data.
• Determined molecular formula using empirical formula and molar mass.

Ensure completion of all problems on the worksheet and homework for better understanding. Prepare for Chapter 7.