Chapter 9: Solutions – Key Concepts and Calculations

Chapter 9: Solutions

9.1 Solutions

Solutions are homogeneous mixtures composed of two or more substances. In a solution, the solute is the component present in a lesser quantity, while the solvent is present in the largest quantity. When water is the solvent, the solution is called an aqueous solution. Solutions are clear, may be colored or colorless, and do not scatter light.

• Solute: Substance dissolved in the solvent (lesser quantity).

• Solvent: Substance that dissolves the solute (greater quantity).

• Example: In a mixture of 20 g sugar and 100 g water, water is the solvent and sugar is the solute.

9.2 Electrolytes and Non-electrolytes

When ionic compounds dissolve in water, they dissociate into positive and negative ions, forming electrolyte solutions that conduct electricity. Covalent compounds, formed from non-metals, do not dissociate and form non-electrolyte solutions that do not conduct electricity.

• Electrolytes: Ionic compounds (metal + non-metal/polyatomic ion) that dissociate into ions in water.

• Non-electrolytes: Covalent compounds (non-metals only) that dissolve as neutral molecules.

• Example: NaCl in water dissociates into Na+ and Cl- ions.

9.3 Solubility and Solvation

Solubility refers to the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature. The process of dissolving an ionic solid in water involves the separation of ions from the crystal lattice, surrounded by water molecules—a process called solvation (or hydration in water).

• Solvation: Surrounding of solute ions or molecules by solvent molecules.

• Example: NaCl(s) → Na+(aq) + Cl-(aq) in water.

9.4 Concentration of Solutions: Molarity (M)

Molarity (M) is a common unit of concentration, defined as the number of moles of solute per liter of solution. It is calculated using the formula:

• Formula: $M = \frac{\text{mol solute}}{\text{L solution}}$

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

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

• Example Calculation: What is the molarity of 0.30 mol HCl in 200 mL solution?

  • Convert 200 mL to L: $0.200$ L

  • $M = \frac{0.30}{0.200} = 1.5$ M

9.5 Calculating Molarity and Moles

To calculate the molarity or the number of moles in a solution, use the relationships above. Always ensure the volume is in liters (L).

• Example 1: Calculate the molarity of 2.0 L solution containing 5.0 mol NaOH.

  • $M = \frac{5.0}{2.0} = 2.5$ M

• Example 2: How many moles are in 500 mL of 0.400 M HCl?

  • Convert 500 mL to L: $0.500$ L

  • $\text{mol solute} = 0.400 \times 0.500 = 0.200$ mol

9.6 Properties of Solutions

Solutions are clear and transparent, with no visible particles, and do not scatter light. They may be colored or colorless, depending on the solute. Examples include saline solution, sugar water, and sports drinks.

Practice Problems and Applications

• Balancing Equations: Practice balancing chemical equations, such as $\mathrm{N_2 + H_2 \rightarrow NH_3}$ and $\mathrm{CH_4 + O_2 \rightarrow CO_2 + H_2O}$, to reinforce stoichiometry concepts relevant to solution calculations.

• Identifying Solute and Solvent: In a solution of sugar and water, water is the solvent and sugar is the solute.

• Electrolyte Identification: Ionic compounds (e.g., NaCl, CaCl2) are electrolytes; covalent compounds (e.g., SO2, P2O5) are non-electrolytes.