CA

Moles & Molarity Lecture Notes

Module Objectives

  • Key concepts: mass, moles, molar mass, percent composition, solution concentration, and dilution problems.

The Mole

  • A mole (mol) is a collective unit in chemistry, defined as 1 \text{ mole} = 6.022140857 \times 10^{23} \text{ units} (Avogadro’s Constant). It’s crucial for counting reactive units like atoms and molecules, as mass alone is insufficient for this purpose. Atomic mass is expressed in grams per mole (g/mol).

Conversions

  • The notes cover conversions between atoms and moles, and grams and moles, using Avogadro's Constant and atomic mass (e.g., 1.01928 \text{ mole He} \times \frac{6.022140857 \times 10^{23} \text{ atoms He}}{1 \text{ mole He}} = 6.13825 \times 10^{23} \text{ atoms He} and 1.0000 \text{ g He} \times \frac{1 \text{ mole He}}{4.002602 \text{ g He}} = 0.24984 \text{ moles He}).

Molecular Mass Calculations

  • Molecular mass (molar mass) is the sum of the atomic masses of all atoms in a molecule. Formula: \text{Mol Mass} = \text{(a \times Atomic mass of A)} + \text{(b \times Atomic mass of B)} + \dots

    • Example: For \text{FeCl}_3 it is (1 \times 55.845 \text{ g/mol}) + (3 \times 35.453 \text{ g/mol}) = 162.204 \text{ g/mol} .

Percent Composition

  • Calculated by dividing the total mass of an element in a compound by the compound's molecular mass, then multiplying by 100. For \text{Fe}2\text{S}3 , \frac{111.690 \text{ g Fe}}{207.885 \text{ g}} \times 100 = 53.7268 \text{% Fe} .

Solution Basics & Molarity

  • Solutions have a solvent (larger amount) and solute (smaller amount).

  • Molarity (M) defines concentration as moles of solute per liter of solution: M = \frac{\text{Moles of solute}}{\text{Volume of solution (L)}} .

    • This formula can be rearranged to find moles ( \text{Moles} = M \times L ) or volume ( L = \text{Moles} \div M ).

    • Example: 0.500 moles Li in 1.000 L is 0.500 M.

Dilution Concept

  • Dilution problems use the formula M1V1 = M2V2 . This allows calculating unknown concentrations or volumes during dilution.

    • For example, to make 500.0 mL of a 0.1250 M solution from a 2.000 M solution, V_1 = \frac{0.1250 \text{ M} \times 500.0 \text{ mL}}{2.000 \text{ M}} = 31.25 \text{ mL} .

Other Concentration Expressions

  • Other concentration types like mass-to-mass, volume-to-volume, and mass-to-volume percent concentrations are also noted as context-dependent.