chem midterms (pt.3)

Quantitative Chemistry

  1. Ways of Expressing Concentration of Solutions

Learning Points

  • Define various ways of describing solution composition.

  • Use different ways of expressing concentration of solutions:

    • Percent by mass

    • Mole fraction

    • Molarity

    • Molality

    • Percent by volume

    • Parts per million (ppm)

  • Solve problems on concentration of solutions.

  • Explain the importance of identifying the concentration of solutions.

Importance During COVID-19

  • Isopropyl alcohol manufacturers maximizing capacity due to surge in demand for sanitizers and cleaning products.

  • Governments and manufacturers need to find ways to make essential materials available.

Understanding Alcohol Concentration

  • Varieties of Isopropyl Alcohol concentrations: 40%, 70%, 99%, and 100%.

  • For disinfecting purposes in most applications, a 70% solution is preferable over a 40%.

  • Different applications require different concentrations (e.g., cleaning electronics, disinfecting surfaces).

What is Concentration?

  • Definition: A measure of how much of a substance is mixed with another substance.

  • Solutions can be classified as dilute or concentrated.

Concentrated vs. Dilute Solutions

Concentrated Solution

  • Characterized by a large amount of solute in a given amount of solvent.

Dilute Solution

  • Characterized by a small amount of solute in a given amount of solvent.

Components of a Solution

  • Two components: solute and solvent.

    • Solute: Present in smaller amount, dissolves in the solvent.

    • Solvent: Present in greater amount, dissolves the solute.

Examples

  • Coffee Solution:

    • Caffeine: Solute

    • Water: Solvent

  • Vinegar Solution:

    • Acetic acid: Solute

    • Water: Solvent

  • Ocean Water:

    • Salt: Solute

    • Water: Solvent

Qualitative vs. Quantitative Concentration

  • Terms "concentrated" and "dilute" provide qualitative descriptions.

  • Quantitative measurements are essential in chemistry for precision.

Calculating Percentage Concentration by Mass

Formula

  • Percentage Concentration (% by mass) = (Mass of solute / Mass of solution) × 100

Example Problems

  1. Pure Gold:

    • 18 karats contains 18 g of gold in 24 g of material.

    • % = (18 g / 24 g) × 100 = 75%.

  2. Potassium Sulfate Solution:

    • 0.49 g in 12.70 g water.

    • % = (0.49 g / (0.49 g + 12.70 g)) × 100 = 3.70%.

  3. Commercial Bleach Solution:

    • 5.25% by mass in 245 grams of solution.

    • Mass of solute = (5.25% × 245 g) / 100 = 12.9 g.

    • Mass of solvent = 245 g - 12.9 g = 232.1 g.

Calculating Percentage Concentration by Volume

Formula

  • Percentage Concentration (% by volume) = (Volume of solute / Volume of solution) × 100

Examples

  1. Ethanol Solution:

    • 25 mL ethanol in 200 mL solution.

    • % by volume = (25 mL / 200 mL) × 100 = 12.5%.

  2. Hydrogen Peroxide:

    • 90 mL in 3000 mL solution.

    • % by volume = (90 mL / 3000 mL) × 100 = 3%.

Molarity, Molality, and Mole Fraction

Molarity (M)

  • Number of moles of solute per liter of solution.

Molality (m)

  • Number of moles of solute per kilogram of solvent.

  • Example: 18.0 g of C6H12O6 in 1 kg water.

Mole Fraction (X)

  • Number of moles of one component divided by total moles in solution.

  • Xsolute + Xsolvent = 1.

Stoichiometric Calculations for Reactions in Solution

  • Concentration of solutions is crucial for stoichiometry of chemical reactions.

  • Balanced equations are essential for mole-to-mole relationships.

Mole Ratios

  • Reactant and product relationships indicated by coefficients in balanced equations.

Examples

  1. Mole-to-Mole Conversion:

    • 4.20 moles of hydrogen reacting with nitrogen.

  2. Mole-to-Mass Conversion:

    • Determining mass of NaCl produced from chlorine gas.

Colligative Properties of Solutions

Definition

  • Properties depend on the number of solute particles in solution (e.g., vapor-pressure lowering, boiling point elevation).

Examples of Colligative Properties

  1. Vapor Pressure Lowering: The vapor pressure of a solution is lower than that of its solvent due to solute presence.

  2. Boiling Point Elevation: Solutions boil at higher temperatures than pure solvents.

  3. Freezing Point Depression: The freezing point of solutions is lower than that of pure solvents.

  4. Osmotic Pressure: Pressure needed to prevent osmosis across semipermeable membranes.

Applications

  • Use of salt on ice in ice cream making versus de-icing roads.

  • Importance in food preservation (jam making) to inhibit bacterial growth.