Chapter 5: Introduction to Solutions and Aqueous Reactions – Study Notes

Introduction to Solutions and Aqueous Reactions

Solution Concentration: Definitions and Types

Solutions are homogeneous mixtures composed of two or more substances. Understanding the terminology and classification of solutions is essential for studying their properties and behavior in chemical reactions.

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

• Solute: The substance(s) present in the smaller amount(s).

• Solvent: The substance present in the larger amount.

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

Examples:

Solution	Solvent	Solute
Soft drink (l)	H2O	Sugar, CO2
Air (g)	N2	O2, Ar, CH4
Solder (s)	Pb	Sn

Concentrated and Dilute Solutions

Solutions can be described quantitatively as either dilute or concentrated, depending on the relative amount of solute present.

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

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

Illustration: In a dilute solution, solute particles are sparsely distributed, while in a concentrated solution, they are more densely packed.

Quantifying Solution Composition

Because solutions are mixtures, their composition can vary from one sample to another. To compare and analyze solutions, chemists quantify the amount of solute relative to solvent using the concentration of solution.

Solution Concentration: Molarity

Definition and Formula

Molarity (M) is the most common way to express solution concentration. It is defined as the amount of solute (in moles) divided by the volume of solution (in liters).

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

Preparing a Solution of Specified Molarity

To prepare a solution of a specific molarity, follow these steps:

1. Weigh out the required amount of solute (in grams).

2. Add the solute to a volumetric flask.

3. Add water and dissolve the solute completely.

4. Fill the flask with water up to the desired volume mark.

5. Mix thoroughly to ensure uniform concentration.

Example: Preparing 1 L of 1.00 M NaCl solution requires dissolving 58.44 g of NaCl in enough water to make 1.00 L of solution.

Using Molarity in Calculations

Molarity serves as a conversion factor between moles of solute and liters of solution. This is useful for stoichiometric calculations in chemical reactions.

• For a 0.500 M NaCl solution: $0.500 \text{ mol NaCl}$ per $1 \text{ L solution}$

• Conversion factors:

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

  • $1 \text{ mol NaCl} \rightarrow 2.00 \text{ L solution}$

Example Calculation: Molarity

To find the molarity of a solution containing 25.5 g KBr dissolved in enough water to make 1.75 L of solution:

• Step 1: Convert grams of KBr to moles using molar mass ($119.00 \text{ g/mol}$): $25.5 \text{ g KBr} \times \frac{1 \text{ mol KBr}}{119.00 \text{ g KBr}} = 0.21429 \text{ mol KBr}$

• Step 2: Calculate molarity: $\text{Molarity (M)} = \frac{0.21429 \text{ mol KBr}}{1.75 \text{ L solution}} = 0.122 \text{ M}$

Example: A solution with 25.5 g KBr in 1.75 L water has a molarity of 0.122 M.

Additional info:

• Molarity is temperature-dependent because solution volume can change with temperature.

• Other units of concentration include molality, percent composition, and parts per million (ppm), but molarity is most common in introductory chemistry.