Aqueous Solutions Notes

Module 13: Aqueous Solutions Overview

  • Solution: A homogeneous mixture of 2 or more substances.
  • Solute: Part of a solution present in lower concentration.
  • Solvent: Part of the solution present in higher concentration.
  • Aqueous solution: A solution where water is the solvent.

Concentration

  • Definition: Relative amount of a given solution component.
  • Dilute vs. Concentrated: Terms describing low vs. high concentration.

Preparing a Solution

  • Example: Preparing a solution of Cobalt(II) chloride dihydrate (CoCl2.2H₂O).
  1. Weigh out the required mass of solute (10.0 g).
  2. Add a portion of the solvent to a volumetric flask.
  3. Swirl to dissolve the solute.
  4. Add additional solvent to reach the desired volume (500 mL).

Dissociation

  • Dissociation: The process where ionic compounds break into ions when dissolved in water.
  • Example with NaCl:
    • NaCl(s) → Na⁺(aq) + Cl⁻(aq)
  • Resulting solutions contain ions and are classified as strong electrolytes.

Concentration Units

  • Molarity (M): Molarity is the number of moles of solute per liter of solution.
  • Formula: M = moles of solute / liters of solution.

Practice Problems

  • Example 1: Calculate the molarity of a solution when 0.122 mol of KI is dissolved in 115 mL of water.

  • Molarity = 0.122 mol / 0.115 L = 1.06 M KI

  • Example 2: Calculate the molarity of a solution prepared by dissolving 5.7 g NaF (molar mass = 42 g/mol) in 2.0 L of water.

  • Steps:

    1. Calculate moles: 5.7 g / 42 g/mol = 0.136 mol.
    2. Molarity = 0.136 mol / 2.0 L = 0.068 M.

  • Example 3: Determine the mass required for 155.0 mL of a 1.30 M Cu(NO3)2 solution.

  • Steps:

    1. Calculate moles: 0.1550 L × 1.30 mol/L = 0.2015 mol.
    2. Convert to grams: 0.2015 mol × 187.56 g/mol = 37.8 g.

Dilution of Solutions

  • Dilution: Process that reduces the concentration of a solution by adding solvent.
  • Stock solution: A concentrated solution prepared for dilution.
  • Dilution Equation: C1V1 = C2V2 (where C is concentration and V is volume).

Concentration Units Variations

  • Mass Percent: Ratio of solute mass to solution mass expressed as a percentage.
  • Parts per Million (ppm) and Parts per Billion (ppb): Used for very low concentrations.
  • ppm = (mass of solute / mass of solution) × 10^6
  • ppb = (mass of solute / mass of solution) × 10^9

Additional Practice Problems

  1. Mass Percent Calculation: Calculate how much water must be added to prepare an 8.15% by mass solution from 5.00 g of urea.
  2. Finding Molarity: If a hazardous chemical bottle is labeled 20.2% by mass of HCl with a density of 1.096 g/mL, calculate its molarity.

Summary of Key Formulas

  • Molarity (M) = moles of solute / volume of solution (L)
  • Mass Percent (%) = (mass of solute / mass of solution) × 100%
  • Volume Percent (%) = (volume of solute / volume of solution) × 100%
  • ppm = (mass of solute / mass of solution) × 10^6
  • ppb = (mass of solute / mass of solution) × 10^9

Tips for Exam Preparation

  • Focus on understanding concepts of solute, solvent, and solution.
  • Practice calculating molarity and understanding dilution.
  • Familiarize yourself with various units of concentration and how to convert between them.