Chapter 3: Composition of Substances and Solutions

Introduction

• Chapter 3 and 4 are heavily math-focused in general chemistry.
• Dimensional analysis from Chapter 1 is crucial.
• Main question: how do we calculate and perform math with molecular compositions?

Subchapters

1. 3.1 Formula Mass and the Mole Concept
2. 3.2 Empirical and Molecular Formulas
3. 3.3 Molarity in Solutions
4. 3.4 Other Units

3.1 Formula Mass and Mole Concept

• Formula Mass: Sum of average atomic masses of all atoms in a compound.
  • Atomic mass = mass of one atom
  • Formula mass (or molecular mass) = mass of a molecule or covalent compound.
• Example: Chloroform (CHCl₃)
  • Formula: 1 Carbon, 1 Hydrogen, 3 Chlorines
  • Calculate: Add atomic masses (C: 1(12), H: 1(1), Cl: 3(35).)
  • Result: 119.37 amu (atomic mass units)

Calculating Formula Mass

• Aspirin (C₉H₈O₄)
  • Formula: 9 Carbons, 8 Hydrogens, 4 Oxygens
  • Calculate: Sum of each element’s atomic masses
  • Result: 180.15 amu
• Ionic Compounds
  • Calculate similarly to molecular mass but termed as formula mass
  • Example: Sodium Chloride (NaCl)

The Mole Concept

• Mole: Amount of substance; Avogadro's number = 6.022 × 10²³ units/mole.
• Converting Mass and Moles:
  • Molar mass (g/mol) is numerically equal to formula mass (amu).
  • Formula mass helps in calculating molar mass.
  • Examples:
    • 1 mole of Zinc = 65.4 g
    • 1 mole of Calcium = 40.08 g
• Units: Grams per mole (g/mol)

Mathematical Relationships

• Conversions:
  • Grams to moles using molar mass.
  • Moles to atoms using Avogadro's number.
• Dimensional Analysis: Critical for tracking units and ensuring correct calculations.

Practice Problems

• Convert grams to moles and vice versa.
• Calculate formula mass for molecules and ionic compounds.
• Example problems provided for hands-on practice (e.g., Potassium, Argon).

Key Takeaways

• Conversion Factors: Use them to relate mass, moles, and number of atoms.
• Molar Mass and Formula Mass: Numerically equal but used in different contexts (atomic vs macroscopic scale).
• Consistent Practice: Essential for mastering concepts in chemistry.