AP

Recording-2025-03-31T04:19:40.370Z

Reactions in Solution

  • Many chemical reactions occur in solution form.

  • This section extends stoichiometry concepts to reactions involving solutions.

Key Concepts

  • Stoichiometric Calculations: Ability to perform calculations based on the amounts of reactants and products.

  • Balanced Equations: Coefficients indicate the number of moles of reactants/products.

  • Converting to Moles: When using pure substances, mass is converted to moles using molar mass. However, in solutions:

    • Molarity (M): Defined as moles of solute per liter of solution.

    • Formula: M = moles of solute / liters of solution.

Approach to Stoichiometry Involving Solutions

  • Utilize molarity (concentration) and volume to find moles:

    • Equation: Moles of solute = Molarity (M) × Volume (L).

  • Reference Figure 4.15 for the procedure of solving stoichiometry problems between a pure substance and a solution.

Example: Neutralization Reaction

  • Problem: Calculate grams of Ca(OH)₂ needed to neutralize 25 mL of 0.1 M HNO₃.

  • Reactants: HNO₃ (acid) and Ca(OH)₂ (base).

  • Steps:

    1. Use molarity and volume of HNO₃ to find moles of HNO₃.

    2. Use mole ratios from the balanced chemical equation to find moles of Ca(OH)₂ required.

    3. Convert moles of Ca(OH)₂ to grams using its molar mass.

Titration

  • Purpose: Determine the concentration of an unknown solution (solute) using a known standard solution (reagent).

  • Equivalence Point: The stage at which the amount of added standard solution exactly reacts with the unknown solution.

  • Common Techniques: Neutralization, precipitation, or oxidation-reduction reactions.

Neutralization Titration Example

  • Example Setup: HCl (unknown concentration) titrated with NaOH (standard solution).

  • Procedure:

    1. Measure a specific volume of HCl solution and add it to a flask.

    2. Add an acid-base indicator that changes color at the equivalence point (e.g., phenolphthalein).

    3. Gradually add NaOH from a burette until the solution turns pink, indicating completion of the reaction.

    4. Record the volume of NaOH added to calculate the concentration of HCl using the volumes and concentrations of both solutions.

Example: Concentration Calculation from Titration

  • Given: 45.7 mL of 0.5 M H₂SO₄ neutralizes 20 mL of NaOH solution.

  • Steps:

    1. Calculate moles of H₂SO₄ using its volume and molarity.

    2. Use the balanced chemical equation to determine moles of NaOH.

    3. Calculate NaOH molarity using its moles and volume.

  • Note: Always ensure units are consistent (e.g., convert mL to L where necessary).

Solving Moles of H₂SO₄

  • Calculation Formula:

    • Moles H₂SO₄ = (45.7 mL) × (0.5 M) × (1 L / 1000 mL).

  • Follow similar procedures for calculating moles of NaOH and deriving its concentration.

  • Balanced Equation Consideration: Always refer to the chemical equation to ensure proper stoichiometric factors are used.