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.