Chapter 8: Introduction to Solutions and Aqueous Reactions – Study Notes

Solution Concentration and Solution Stoichiometry

Definitions and Properties of Solutions

Solutions are homogeneous mixtures in which the composition is uniform throughout. When substances like table salt (NaCl) are mixed with water, they dissolve and form a solution, but the solute remains present and can be recovered by removing the solvent.

• Solution: A homogeneous mixture of two or more substances.

• Solute: The minor component in a solution (the substance being dissolved).

• Solvent: The major component in a solution (the substance doing the dissolving).

Categories of Solution Concentration

Solutions are described as dilute or concentrated based on the relative amount of solute to solvent.

• Dilute solution: Contains a small amount of solute compared to solvent.

• Concentrated solution: Contains a large amount of solute compared to solvent.

Molarity (M) – Quantitative Expression of Concentration

Molarity is the most common unit for expressing solution concentration in chemistry. It is defined as the number of moles of solute per liter of solution.

• Formula:

$ext{Molarity (M)} = \frac{\text{amount of solute (in mol)}}{\text{volume of solution (in L)}}$

Using Molarity in Calculations

Molarity serves as a conversion factor between moles of solute and volume of solution. This is essential for stoichiometric calculations in chemical reactions involving solutions.

• Example: A 0.500 M NaCl solution contains 0.500 mol NaCl per liter of solution.

• Conversion factors:

  • $0.500 \text{ mol NaCl} / 1 \text{ L solution}$

  • $1 \text{ L solution} / 0.500 \text{ mol NaCl}$

Preparing Solutions of Specific Concentration

To prepare a solution of known molarity, weigh the required amount of solute, dissolve in water, and dilute to the desired final volume.

• Example: To prepare 1 L of 1.00 M NaCl, dissolve 58.44 g NaCl in water and dilute to 1.00 L.

Solution Dilution: Making a Solution from a Stock Solution

Dilution involves preparing a less concentrated solution from a more concentrated stock solution by adding solvent. The amount of solute remains constant; only the volume changes.

• Key equation: $C_1V_1 = C_2V_2$ or $M_1V_1 = M_2V_2$

• $C_1$ and $V_1$ are the concentration and volume of the stock solution; $C_2$ and $V_2$ are those of the diluted solution.

Solution Stoichiometry

Stoichiometry in solutions uses molarity to relate the volume of solution to the amount of solute, which is crucial for quantitative chemical reactions.

• Formula: $\text{Molarity of A} \times \text{Volume of A} = \text{moles of A}$

• Example: $0.0200 \text{ L} \times 0.50 \text{ mol/L NaCl} = 0.010 \text{ mol NaCl}$

Types of Aqueous Solutions and Solubility

Solubility and Dissolution

Solubility describes how well a solute dissolves in a solvent. "Like dissolves like" is a guiding principle: polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents.

• Salt water: NaCl (ionic) dissolved in H2O (polar)

• Sugar water: C12H22O11 (polar) dissolved in H2O (polar)

What Happens When a Solute Dissolves?

Dissolution involves breaking solute-solute and solvent-solvent interactions and forming solute-solvent interactions. If the new interactions are strong enough, the solute dissolves.

• Attractive forces between solute particles and solvent molecules are key.

Charge Distribution in Water Molecule

Water is a polar molecule with an uneven distribution of electrons, resulting in a partial negative charge on oxygen and a partial positive charge on hydrogen.

Solute and Solvent Interactions in Ionic Solutions

When ionic compounds like NaCl dissolve in water, the ions are separated and surrounded by water molecules, forming a solution with free-moving charged particles.

• These solutions can conduct electricity.

Electrolyte and Nonelectrolyte Solutions

Definitions

• Electrolytes: Substances that dissolve in water to form solutions that conduct electricity (due to the presence of ions).

• Nonelectrolytes: Substances that dissolve in water but do not conduct electricity (no ions present).

Types of Electrolytes

• Strong electrolytes: Completely dissociate into ions (e.g., NaCl, CaCl2).

• Weak electrolytes: Partially dissociate into ions (e.g., CH3COOH).

• Nonelectrolytes: Dissolve as intact molecules (e.g., sugar).

Electrolytic Properties of Solutions

Solution	Type	Conductivity
NaCl(aq)	Strong Electrolyte	High
HC2H3O2(aq)	Weak Electrolyte	Low
C12H22O11(aq)	Nonelectrolyte	None

Solubility of Ionic Compounds

General Principles

When ionic compounds dissolve, their ions separate and disperse in water. Not all ionic compounds are soluble; some remain as solids.

• Soluble: Dissolves in water (e.g., AgNO3).

• Insoluble: Does not dissolve in water (e.g., AgCl).

Solubility Rules

Solubility rules help predict whether an ionic compound will dissolve in water.

Compounds Containing the Following Ions are Generally Soluble	Exceptions
Li+, Na+, K+, NH4+	None
NO3-, C2H3O2-	None
Cl-, Br-, I-	Ag+, Hg22+, Pb2+
SO42-	Ag+, Ca2+, Sr2+, Ba2+, Pb2+

Compounds Containing the Following Ions are Generally Insoluble	Exceptions
OH-	Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+
S2-	Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+
CO32-, PO43-	Li+, Na+, K+, NH4+

Precipitation Reactions

Definition and Process

Precipitation reactions occur when two aqueous solutions of ionic compounds are mixed and an insoluble ionic compound (precipitate) forms.

• Precipitate: The insoluble product formed in a precipitation reaction.

• Example: $2KI(aq) + Pb(NO_3)_2(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$

Predicting Precipitation Reactions

1. Identify the ions present in each reactant.

2. Exchange ions to determine possible products.

3. Use solubility rules to determine if a product is insoluble (will precipitate).

4. If no product is insoluble, write "no reaction."

5. Balance the chemical equation.

Summary Table: Steps for Predicting Precipitation Reactions

Step	Description
1	Identify ions in reactants
2	Exchange ions to form products
3	Determine solubility of products
4	Write (s) for precipitate, (aq) for soluble
5	Balance the equation

Additional info:

• Periodic Table is included for reference in stoichiometric and solubility calculations.

• Practice problems are provided throughout to reinforce concepts and calculation methods.