Honors Chemistry Solutions

Definition of Solutions

Solution: A homogeneous mixture that is physically combined.
  - Examples:
    - Lemonade (liquid)
    - Metal alloys (solid)
    - Air (gas)

Solvent vs. Solute

Solvent: The dissolving medium, which is present in a greater amount.
- Example: Water
Solute: The dissolving particles, present in a lesser amount.
- Example: Salt
Dissolution Process: The solvent dissolves the solute.
- Example: In soda water, Carbon Dioxide is the solute dissolved in Water (the solvent).

Process of Dissolving

For dissolving to occur, the solute must come into contact with the solvent and…
Collision: The solute particles must collide with solvent particles for dissolution to happen.

Factors Affecting Solubility

Stirring
- Increases contact between solute and solvent, thereby increasing the number of collisions.
Temperature
- Higher temperature signifies more kinetic energy, resulting in faster-moving molecules which collide more often.
Particle Size
- Smaller particles provide a greater surface area, allowing more solute particles to be exposed for collisions.

Saturation Levels in Solutions

Saturated Solution: Contains the maximum amount of solute at a given temperature; no more solute can be dissolved.
Unsaturated Solution: Contains less solute than it can hold at a given temperature; can dissolve more solute; considered a "dilute" solution.
Supersaturated Solution: Contains more solute than it can normally hold at a given temperature.

Solubility

Solubility: The amount of solute that dissolves completely in a given quantity of solvent at a specific temperature and pressure, often expressed in grams of solute per 100 g of solvent.

Solubility Rule: "Like Dissolves Like"

Polar Substances: Dissolve in water; all ionic compounds are polar and have charges that interact with water.
Nonpolar Substances: Do not dissolve in water; examples include oils.

Solubility Curves

Below the line on a solubility curve: Unsaturated
On the line: Saturated
Above the line: Supersaturated
- If dissolved: Supersaturated.
- If not dissolved: Saturated with undissolved solute.

Example Problems

If 60 grams of KNO3 are dissolved in 100 g of water at 50°C, classify the solution as unsaturated, saturated, or supersaturated.
Determine how many more grams can be dissolved at this temperature.
A saturated solution of KCl in 750 grams of water at 80 °C contains how much solute?
If 95 grams of sodium nitrate are dissolved in 100 grams of water at 20 °C, identify the type of solution.
What temperature must the solution be heated to completely dissolve the solute?

Solutions Vocabulary

Aqueous: Water that contains dissolved substances.
Gaseous: A solution that contains gas particles.
Hydrate: A compound that contains water.
Suspension: A mixture from which particles settle out upon standing; suspended particles can be removed by filtration.

Real Life Example

Filtering the Blood: The kidneys filter blood plasma, separating it from white blood cells and platelets.

Warm-up Questions

How many grams of potassium nitrate are needed to make a 155 mL saturated solution at 40°C?
To what temperature does the solution need to be heated to dissolve 30 additional grams?

Answers to Warm-up Questions

153 g and 45°C.

Molarity

Molarity: The number of moles of solute dissolved in one liter of solution; it quantitatively expresses the concentration of a solution.
Terms "dilute" and "concentrated" are not precise.
Molarity (M) = moles of solute / liters of solution
Units: Molar (M), with the following conversions to keep in mind:
- 1 liter = 1 dm³ = 1000 mL = 1000 cm³

Molarity Examples

Example #2: If 7.55 grams of sodium sulfate are dissolved in 750 mL of solution, what is the Molarity?
- Answer: 0.0709 M.
Example #1: If 42.5 grams of potassium chloride are dissolved in 0.25 L of solution, what is the Molarity of this solution?
- Answer: 2.3 M.

Preparing a Solution

To prepare 475 mL of a 3.1 M potassium nitrate solution:
- Use the formula: grams = M x L x Molar Mass.
- Answer: Add 150 grams of KNO3 to 475 mL of water.

Warm-up Questions

To prepare 675 mL of a 0.450 M calcium oxalate solution, how many grams of calcium oxalate are needed?
Distinguish between solute and solvent.

Answer to Warm-up

Answer: Add 38.9 grams of calcium oxalate to 675 mL of water to prepare a 0.450 M solution.

Molarity Calculation Question

What volume of solution is needed to make a 3.4 M aluminum oxalate solution containing 2.65 moles of solute?

Colligative Properties of Solutions

Colligative properties depend only on the amount of solute present.
More solute leads to a greater effect.
  - Types:
    - Boiling point elevation (BPE)
    - Freezing point depression (FPD)
    - Vapor pressure lowering

Effects of Solute on Intermolecular Forces

Adding solute increases the intermolecular forces due to attraction between solute and solvent molecules.

Applications of Colligative Properties

Freezing Point Depression: Adding rock salt to ice when making ice cream lowers the freezing temperature, keeping the ice frozen.
Boiling Point Elevation: Adding salt to water for cooking increases the boiling point of the water.

Key Formulas

Molality (m) = moles of solute / kilograms of solvent
Change in boiling point:
- $riangle T_b = i imes K_b imes m$
Change in freezing point:
- $riangle T_f = i imes K_f imes m$
- Where $i$ = van 't Hoff factor (number of ions from the solute).

Example Problem

Calculate the final boiling point of a sucrose solution containing 5.70 grams of C12H22O11 in 50.0 grams of water.
(Molar mass of sucrose = 342 g/mol)
Final temperature: 100.170°C.

Final Homework Questions

Calculate the final freezing and boiling points of 13.6 grams of potassium nitride in 785 grams of water.
Calculate the final boiling point of 7.5 grams of sucrose in 998 grams of water.
What is the final freezing point of 23.7 grams of calcium chloride in 1090 grams of water?