Chapter 13: Solutions – Study Notes for Introductory Chemistry

Solutions

Concept of Solutions

Solutions are homogeneous mixtures composed of two or more substances that form a uniform composition throughout. The main components of a solution are the solute and the solvent.

• Solute: The substance present in a lesser amount, which is dissolved in the solvent.

• Solvent: The substance present in the greater amount, which dissolves the solute.

• Concentration: A measure of the amount of solute in a given quantity of solvent or solution.

Example: If 10.0 g NaCl is dissolved in 500.0 mL of water, NaCl is the solute and water is the solvent.

Types of Mixtures: Solutions, Suspensions, and Colloids

Mixtures can be classified based on the size of the solute particles and their behavior in the solvent.

• Solutions: Particle size < 1 nm; particles do not settle; homogeneous (e.g., salt water).

• Colloids: Particle size between 1 nm and 500 nm; particles do not settle; appear cloudy (e.g., milk).

• Suspensions: Particle size > 500 nm; particles settle out upon standing; heterogeneous (e.g., salad dressing).

Solubility and Intermolecular Forces

Like Dissolves Like Principle

Solubility depends on the nature of the solute and solvent. According to the principle of like dissolves like, substances with similar intermolecular forces or polarity will dissolve in each other.

• Polar solutes dissolve in polar solvents (e.g., NaCl in water).

• Nonpolar solutes dissolve in nonpolar solvents (e.g., oil in hexane).

Example: Methanol (CH3OH, polar) will dissolve in water (polar), but not in hexane (nonpolar).

Concentration Units

Mass Percent

Mass percent expresses the mass of solute as a percentage of the total mass of the solution.

• Formula: $\text{Mass Percent} = \frac{\text{Mass of Solute}}{\text{Total Mass of Solution}} \times 100\%$

Example: A solution with 23.0 g NaOH in 100 g solution is 23.0% NaOH by mass.

Molarity (M)

Molarity is the number of moles of solute per liter of solution. It is a common unit for expressing solution concentration.

• Formula: $\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}$

Example: To prepare a 0.550 M Na3PO4 solution, dissolve the appropriate mass in enough water to make 300.0 mL of solution.

Osmolarity

Definition and Calculation

Osmolarity is the total concentration of all ions in solution, expressed as moles of ions per liter of solution.

• Formula: $\text{Osmolarity} = \text{Number of ions per formula unit} \times \text{Molarity of compound}$

Example: For AlCl3, which dissociates into 4 ions (1 Al3+ and 3 Cl-), a 0.10 M solution has an osmolarity of 0.40 osmol/L.

Solubility and Temperature

Saturated, Unsaturated, and Supersaturated Solutions

The amount of solute that can dissolve in a solvent depends on temperature.

• Saturated solution: Contains the maximum amount of dissolved solute at a given temperature.

• Unsaturated solution: Can dissolve more solute at the same temperature.

• Supersaturated solution: Contains more solute than is stable at that temperature.

Temperature Effects

As temperature increases, the solubility of most solids in liquids increases, while the solubility of gases in liquids decreases.

Henry’s Law

Gas Solubility and Pressure

Henry’s Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

• Formula: $C = kP$ (where C is concentration, k is Henry’s constant, and P is pressure)

Dilutions

Preparation of Dilute Solutions

Dilution involves adding solvent to a solution to decrease its concentration. The relationship is given by:

• Formula: $M_1V_1 = M_2V_2$

• $M_1$ and $V_1$ are the initial molarity and volume; $M_2$ and $V_2$ are after dilution.

Solution Stoichiometry

Stoichiometric Calculations in Solution

Solution stoichiometry involves using molarity and volume to determine the amounts of reactants and products in chemical reactions.

• Steps: Convert given quantity to moles, use mole ratios, convert to desired units.

Molality (m)

Definition and Calculation

Molality is the number of moles of solute per kilogram of solvent. It is used when temperature changes are involved, as it does not depend on volume.

• Formula: $\text{Molality (m)} = \frac{\text{moles of solute}}{\text{kg of solvent}}$

Colligative Properties

Overview

Colligative properties depend on the number of solute particles in solution, not their identity. The four main colligative properties are:

• Boiling Point Elevation

• Freezing Point Depression

• Vapor Pressure Lowering

• Osmotic Pressure

Boiling Point Elevation

Adding a solute to a solvent raises the boiling point of the solvent.

• Formula: $\Delta T_b = i K_b m$

• $i$ = van’t Hoff factor (number of particles formed per formula unit)

• $K_b$ = boiling point elevation constant

• $m$ = molality

Freezing Point Depression

Adding a solute to a solvent lowers the freezing point of the solvent.

• Formula: $\Delta T_f = i K_f m$

• $K_f$ = freezing point depression constant

Osmosis and Osmotic Pressure

Osmosis

Osmosis is the movement of solvent molecules (usually water) across a semipermeable membrane from a region of lower solute concentration to higher solute concentration.

• Semipermeable membrane: Allows solvent but not solute to pass through.

• Osmotic pressure: The pressure required to stop the net flow of solvent.

Tonicity and Red Blood Cells

Tonicity describes the relative concentration of solutes in two solutions separated by a semipermeable membrane.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell (cell swells).

• Isotonic: Equal solute concentration; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell (cell shrinks).

Example: A red blood cell placed in pure water (hypotonic) will swell and may burst (hemolysis).

Additional info: Colligative properties are important in biological and industrial processes, such as intravenous fluid preparation and antifreeze formulation.