In-Depth Notes on Solutions and Concentration Calculations

Solutions Notes

Objectives

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  • Understand key concepts related to solutions including:

  • Definitions of:

    • Solution: A homogeneous mixture composed of two or more substances.

    • Concentration: Measure of the amount of solute in a given volume of solvent or solution.

    • Homogeneous mixture: Composition is uniform throughout.

    • Aqueous solution: Water is the solvent; non-aqueous solution: another solvent is used.

    • Unsaturated solution: More solute can dissolve.

    • Saturated solution: Maximum amount of solute has dissolved.

    • Supersaturated solution: Contains more dissolved solute than in a saturated solution at that temperature.

    • Standard solution: A solution of known concentration.

  • Define types of concentration:

    • Molar concentration (M)

    • Molal concentration (m)

    • Weight percent

    • Volume percent

    • Parts per million (ppm)

    • Parts per billion (ppb)

  • Ability to calculate and prepare solutions and understand dilution procedures.

Concept Map

  • Solutions include:

  • Homogeneous mixtures

  • Concentration types: Molarity, Molality, Weight Percent, Volume Percent, ppm, ppb.

Molar Concentration (M)

  • Definition: Number of moles of solute per liter of solution.

  • Formula: M = n / v

  • Where:

    • M = molarity (mol/L)

    • n = number of moles

    • v = volume (L)

  • Example: Dissolving 40g of NaOH (1 mol) in 1 dm³ gives a 1 M solution.

Molality (m)

  • Definition: Number of moles of solute per kilogram of solvent.

  • Formula: m = moles of solute / mass of solvent (kg)

  • Example: 40g of NaOH in 1 kg of water gives a 1 molal solution.

Weight Percent (wt%)

  • Definition: Mass of solute per 100g of solution.

  • Formula: %wt = (mass of solute / mass of solution) × 100.

  • Example: 10% NaOH solution means 10g NaOH in 100g solution (90g water).

Volume Percent (vol%)

  • Definition: Volume of solute per 100 mL of solution.

  • Formula: %vol = (volume of solute / total volume of solution) × 100.

  • Example: Wine with 12% alcohol means 12 mL alcohol in 100 mL wine.

Parts per Million (ppm) and Parts per Billion (ppb)

  • ppm (by weight): Formula = (mass of solute / mass of solution) × 10^6.

  • ppm (by volume): Formula = (volume of solute / total volume of solution) × 10^6.

  • ppb (by weight): Formula = (mass of solute / mass of solution) × 10^9.

Calculating Concentrations

  • Weight Percent Example: For 8g NaOH in 50g water:

  • %wt = (8.00g / (8.00g + 50.00g)) × 100 = 13.8% NaOH solution.

  • Vinegar Example: Analysis involving molality, molarity, % by volume, ppm, ppb calculations of acetic acid.

Dilutions

  • Essential in labs for reducing concentration of solutions.

  • Steps to perform dilutions:

  • Calculate desired final volume and concentration.

  • Use precise glassware (pipettes for precision, graduated cylinders for approximate).

  • Final dilution calculation maintains amount of solute: M₁V₁ = M₂V₂.

Volume to Volume Dilutions

  • Ratio of solute to final volume. Example of 1:10 dilution:

  • Mix 1 part concentrated solution with 9 parts solvent.

  • Calculation based on M₁V₁ = M₂V₂.

Worked Problems

  • Example 1: Create 5 mL of 0.25 M solution from a 1M solution, finding V₁.

  • Example 2: Total molarity of HCl after mixing two solutions.

Serial Dilutions

  • Gradual dilution, often by a factor of 10, useful for precise concentrations.

  • Technique: take a known volume of stock solution, dilute incrementally; allows small aliquots rather than large quantities.

  • Final concentration calculated based on the dilution factor and number of colonies on plates in microbiological tests.