chem exam 4 (ch12)-Solutions: Properties, Types, and Concentrations

Solutions

Definition and Components

Solutions are homogeneous mixtures composed of two or more substances. The substance present in the smaller amount is called the solute, while the substance present in the greater amount is the solvent. The solute is uniformly dispersed throughout the solvent, resulting in a single phase.

• Solute: The substance dissolved in the solvent (e.g., salt in water).

• Solvent: The substance that dissolves the solute (e.g., water in a saltwater solution).

• Solutions can be solid, liquid, or gas.

Properties of Solutes in Solutions

• Solutes are not visible and cannot be separated by filtration.

• Solutes can be separated by evaporation.

• Solutes may impart color to a solution (e.g., copper(II) sulfate in water).

Types of Solutes and Solvents

Solutes and solvents can be gases, liquids, or solids, leading to various types of solutions (e.g., air, saltwater, brass).

Solute–Solvent Interactions

Polarity and Solubility

The formation of a solution depends on the attractive forces between solute and solvent particles. The rule "like dissolves like" means that polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents.

• Polar solvents (like water) dissolve ionic and polar solutes.

• Nonpolar solvents (like CH2Cl2) dissolve nonpolar solutes.

Formation of Solutions

When a solute dissolves, the attractive forces between solute and solvent must be strong enough to overcome the forces holding the solute particles together and those holding the solvent molecules together.

Dissolution of Ionic Compounds

For ionic compounds like NaCl, the attraction between the ions and water molecules (hydration) must overcome the ionic lattice energy. Each ion becomes surrounded by water molecules, resulting in a solution of free-moving ions.

Electrolytes and Nonelectrolytes

Definitions and Properties

Substances that dissolve in water to form ions and conduct electricity are called electrolytes. Those that dissolve as molecules and do not conduct electricity are nonelectrolytes.

• Strong electrolytes: Completely dissociate into ions (e.g., NaCl).

• Weak electrolytes: Partially dissociate, producing some ions and mostly molecules (e.g., acetic acid).

• Nonelectrolytes: Do not produce ions (e.g., sugar).

Examples and Equations

• Strong electrolyte: $\mathrm{NaCl(s) \xrightarrow{H_2O} Na^+(aq) + Cl^-(aq)}$

• Weak electrolyte: $\mathrm{HF(aq) \rightleftharpoons H^+(aq) + F^-(aq)}$

• Nonelectrolyte: $\mathrm{CH_4O(l) \xrightarrow{H_2O} CH_4O(aq)}$

Solubility

Definition and Factors Affecting Solubility

Solubility is the maximum amount of solute that can dissolve in a specific amount of solvent at a given temperature. Solutions can be unsaturated (more solute can dissolve) or saturated (no more solute dissolves; excess remains undissolved).

• Solubility is usually expressed as grams of solute per 100 grams of solvent.

• Temperature affects solubility: for most solids, solubility increases with temperature; for gases, it decreases.

Soluble and Insoluble Ionic Compounds

Ionic compounds that dissolve in water are called soluble salts; those that do not are insoluble salts.

Solution Concentrations

Concentration Expressions

The concentration of a solution is the amount of solute dissolved in a specific amount of solution. Common units include mass percent, volume percent, mass/volume percent, and molarity.

• Mass percent (m/m): $\mathrm{\frac{\text{grams of solute}}{\text{grams of solution}} \times 100\%}$

• Volume percent (v/v): $\mathrm{\frac{\text{mL of solute}}{\text{mL of solution}} \times 100\%}$

• Mass/volume percent (m/v): $\mathrm{\frac{\text{grams of solute}}{\text{mL of solution}} \times 100\%}$

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

Sample Calculations

• To calculate mass percent: $\mathrm{\frac{30.0\ g\ NaOH}{150.0\ g\ solution} \times 100\% = 20.0\%}$

• To calculate molarity: $\mathrm{\frac{1.50\ mol\ NaCl}{1.00\ L\ solution} = 1.50\ M}$

Dilution of Solutions

Concept and Calculation

Dilution is the process of adding solvent to a solution to decrease its concentration. The amount of solute remains constant before and after dilution. The relationship is given by:

• $\mathrm{C_1V_1 = C_2V_2}$, where $C$ is concentration and $V$ is volume.

Example: To prepare 1000 mL of 35.0% (m/v) dextrose from 50.0% (m/v) stock, use $V_1 = \frac{C_2V_2}{C_1}$.

Summary Table: Classification of Solutes in Aqueous Solutions

Type	Behavior in Water	Electrical Conductivity	Example
Strong Electrolyte	Completely dissociates into ions	Conducts well	NaCl
Weak Electrolyte	Partially dissociates	Conducts weakly	Acetic acid
Nonelectrolyte	No ions formed	Does not conduct	Sucrose

Key Equations

• Mass percent: $\mathrm{\frac{\text{mass of solute}}{\text{mass of solution}} \times 100\%}$

• Volume percent: $\mathrm{\frac{\text{volume of solute}}{\text{volume of solution}} \times 100\%}$

• Mass/volume percent: $\mathrm{\frac{\text{mass of solute}}{\text{volume of solution}} \times 100\%}$

• Molarity: $\mathrm{\frac{\text{moles of solute}}{\text{liters of solution}}}$

• Dilution: $\mathrm{C_1V_1 = C_2V_2}$