31-Dilutions

Introduction

  • Understanding the concepts of molarity, dilution, and chemical reactions is essential for chemistry.

Molarity

  • Definition: Molarity (M) is defined as the number of moles of solute in 1 liter of solution.

    • Formula: [ M = \frac{n}{V} ] where ( n ) is moles of solute and ( V ) is liters of solution.

    • Conversion: To find grams of solute: ( mass = n \times MW ) where ( MW ) = molecular weight.

Dilution

  • Definition: Dilution is the process of reducing the concentration of a solution by adding more solvent.

  • Key Concept: The number of moles of solute remains unchanged during dilution.

    • Formulas:

      • ( n_{initial} = n_{final} )

      • ( V_{initial} \times M_{initial} = V_{final} \times M_{final} )

Calculation Examples

  • Example Problem: What mass of oxalic acid is needed to prepare 50.00 mL of 0.125 M ( C_2H_2O_4 ) ?

    • Molecular Weight: 90.035 g/mol

    • Calculation Steps:

      1. Convert 50.00 mL to Liters: 0.050 L

      2. Calculate moles: ( (0.125 M) \times (0.050 L) = 0.00625 moles )

      3. Convert to grams: ( 0.00625 moles \times 90.035 g/mol = 0.563 g )

Dissolution and Dissociation

  • Dissolving: The dissolution of a solute does not always mean it will dissociate.

  • Nonelectrolytes: Substances that do not ionize in water (e.g., glucose, ethanol).

  • Electrolytes:

    • Strong Electrolytes: Complete ionization (e.g., NaCl).

    • Weak Electrolytes: Partial ionization (e.g., HF, PbCl2).

Chemical Reactions

  • Exchange Reactions: Involves the exchange of atoms or ions between molecules.

    • Types of chemical reactions:

      1. Molecular Partner - dissolution

      2. Atom Exchange - synthesis, decomposition, substitution, metathesis

      3. Proton Transfer - acid-base reactions

      4. Electron Transfer - oxidation-reduction reactions

Net Ionic Equations

  • Overall Ionic Equation: Represents all ions in solution

  • Net Ionic Equation: Shows only the species that participate in the reaction, excluding spectator ions.

Precipitation Reactions

  • Occurs when soluble reactants form an insoluble product (precipitate).

    • Example Reaction: ( Pb(NO_3)_2(aq) + 2 NaI(aq) → PbI_2(s) + 2 NaNO_3(aq) )

Solubility Factors

  • Ionic Solids: Held together by strong ion-ion forces (ionic bonds).

  • Dissolved Ions: Interact with water through ion-dipole forces.

  • Factors influencing solubility:

    1. Ionic Character: Higher ionic character leads to higher solubility.

    2. Ion Charges: Lower charges result in weaker ion-ion attraction.

    3. Molecular Size: Larger ions lead to longer bond lengths and weaker forces.