Chapter 9: Solutions – General Chemistry Study Notes

Chapter 9: Solutions

9.1 Solutions

Solutions are a fundamental concept in chemistry, describing homogeneous mixtures composed of two or more substances. Understanding the nature and properties of solutions is essential for studying chemical reactions and processes in aqueous environments.

• Solution: A homogeneous mixture of two or more substances, where the composition is uniform throughout.

• Solute: The component present in a lesser quantity; the substance being dissolved.

• Solvent: The component present in the largest quantity; the substance doing the dissolving.

• Aqueous solution: A solution in which water is the solvent.

Example: In a solution of salt water, salt is the solute and water is the solvent.

9.2 Electrolytes and Non-electrolytes

When substances dissolve in water, they may form solutions that conduct electricity (electrolytes) or do not conduct electricity (non-electrolytes). The distinction depends on the nature of the solute and its ability to dissociate into ions.

• Ionic compounds (e.g., NaCl) dissociate in water to form positive and negative ions, creating electrolyte solutions that conduct electricity.

• Covalent compounds (e.g., sugar) typically do not dissociate into ions and form non-electrolyte solutions that do not conduct electricity.

• Solvation: The process of surrounding dissolved ions or molecules with solvent molecules. For ionic solids, this involves the separation of ions from the crystal lattice and their stabilization by water molecules.

Example: When NaCl dissolves in water:

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

Table: Electrolytes vs. Non-electrolytes

Type	Example	Conducts Electricity?
Electrolyte	NaCl, KBr	Yes
Non-electrolyte	Glucose, SO2	No

Example Question: Which of the following would form a non-electrolyte solution? Answer: SO2 and P2O5 (covalent compounds).

9.3 General Properties of Solutions

Solutions exhibit several characteristic properties that distinguish them from other types of mixtures.

• Clear and transparent

• No visible particles

• Do not scatter light (unlike colloids)

• May be colored (e.g., copper sulfate solution) or colorless

9.4 Concentration of Solutions: Molarity

Molarity (M) is a common unit of concentration in chemistry, defined as the number of moles of solute per liter of solution.

• Formula:

$\mathrm{Molarity\ (M) = \frac{\text{mol\ solute}}{\text{L\ solution}}}$

• Unit: M (moles per liter, or mol/L)

• To find moles of solute: $\mathrm{mol\ solute = M \times L\ solution}$

• To find volume: $\mathrm{L\ solution = \frac{mol\ solute}{M}}$

Example Calculation: What is the molarity of 2.0 L of solution containing 5.0 mol NaOH?

$\mathrm{M = \frac{5.0\ mol}{2.0\ L} = 2.5\ M}$

9.5 Calculating Molarity and Moles

To solve problems involving molarity, always ensure the volume is in liters (L). If given in milliliters (mL), divide by 1000 to convert to liters.

• Example 1: 200 mL solution with 0.30 mol HCl:

Convert 200 mL to L: $200\,\text{mL} \div 1000 = 0.200\,\text{L}$

$\mathrm{M = \frac{0.30\ mol}{0.200\ L} = 1.5\ M}$

• Example 2: 4.50 mol NaF in 7.50 L solution:

$\mathrm{M = \frac{4.50\ mol}{7.50\ L} = 0.600\ M}$

• Example 3: How many moles of NH3 are present in 1.50 L of a 0.500 M NH3 solution?

$\mathrm{mol\ solute = 0.500\ M \times 1.50\ L = 0.750\ mol}$

• Example 4: Number of moles in 500 mL of 0.400 M HCl:

Convert 500 mL to L: $500\,\text{mL} \div 1000 = 0.500\,\text{L}$

$\mathrm{mol\ solute = 0.400\ M \times 0.500\ L = 0.200\ mol}$

9.6 Balancing Chemical Equations (Review)

Balancing chemical equations is essential for representing chemical reactions accurately. The number of atoms of each element must be the same on both sides of the equation.

• Example 1: $\mathrm{N_2 + H_2 \rightarrow NH_3}$

• Balanced: $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$

• Coefficients: 1 (N2), 3 (H2), 2 (NH3)

• Example 2: $\mathrm{CH_4 + O_2 \rightarrow CO_2 + H_2O}$

• Balanced: $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$

• Coefficients: 1 (CH4), 2 (O2), 1 (CO2), 2 (H2O)

Steps to Balance Equations:

1. Write the unbalanced equation.

2. Count the number of atoms of each element on both sides.

3. Add coefficients to balance each element.

4. Check your work to ensure all elements are balanced.

Summary Table: Key Terms and Definitions

Term	Definition
Solution	Homogeneous mixture of two or more substances
Solute	Component present in lesser quantity
Solvent	Component present in largest quantity
Electrolyte	Substance that forms ions in solution and conducts electricity
Non-electrolyte	Substance that does not form ions in solution and does not conduct electricity
Molarity (M)	Moles of solute per liter of solution
Solvation	Process of surrounding solute particles with solvent molecules

Additional info: These notes cover the foundational aspects of solutions, electrolytes, and concentration calculations, as well as a brief review of balancing chemical equations, all of which are essential for General Chemistry students.