Solution Concentration, Preparation, and Dilution in General Chemistry

Learning Objectives and Assignments

Overview

• Estimate concentration of solute in solutions

• Convert between moles, mass, and solutions

• Determine solute concentration in solutions

• Describe energy changes associated with chemical reactions

• State the first law of thermodynamics

Assignments:

• Reading for Today: Textbook sections 4.5–4.6

• Reading for Friday: Textbook sections 5.2–5.4

• Problem Set #4 – Chapter 4 (due Next Wed. Oct. 15)

Chapter 4: Reactions in Aqueous Solutions

Section 4.5 – Concentrations of Solutions

Understanding solution concentration is essential for chemists to quantify the amount of solute dissolved in a solvent. The most common way to express concentration is molarity.

• Concentration: The amount of solute present in a given quantity of solution.

• Molarity (M): Defined as the number of moles of solute per liter of solution.

Formula:

$ext{Molarity}~(M) = rac{ ext{moles of solute}}{ ext{volume of solution in liters}}$

• Molarity can be used as a conversion factor between moles and liters in chemical calculations.

Molarity Calculations

Example Calculation

To find the molarity of a solution, you need the mass of solute, its molar mass, and the final volume of solution.

• Step 1: Calculate moles of solute using its mass and molar mass.

• Step 2: Convert solution volume to liters.

• Step 3: Apply the molarity formula.

Example: Dissolving 3.68 g of sucrose ($C_{12}H_{22}O_{11}$) in 275.0 mL water:

• Moles of sucrose: $3.68~g imes rac{1~mol}{342~g} = 0.0105~mol$

• Volume in liters: $275.0~mL imes rac{1~L}{1000~mL} = 0.275~L$

• Molarity: $M = rac{0.0105~mol}{0.275~L} = 0.0382~M$

Mixing a Solution

Preparation of Standard Solutions

To prepare a solution of known molarity:

1. Weigh out a known mass of solute.

2. Add solute to a volumetric flask.

3. Add solvent up to the calibration mark on the flask neck.

Example: Preparing 250 mL of 0.100 M $CuSO_4$ solution by weighing 3.99 g $CuSO_4$ and dissolving in water.

Dilution of Solutions

Principles of Dilution

A solution can be diluted by adding only solvent. The total number of moles of solute remains unchanged, but the concentration decreases.

• Key Equation: $M_1V_1 = M_2V_2$

• $M_1$ and $V_1$ are the molarity and volume of the concentrated solution; $M_2$ and $V_2$ are those of the diluted solution.

Example: To make 250 mL of 0.100 M solution from 1.00 M stock:

• $V_1 = rac{M_2V_2}{M_1} = rac{0.100~M imes 250~mL}{1.00~M} = 25~mL$

Summary Table: Solution Preparation and Dilution

Step	Description
Weigh Solute	Determine mass needed for desired moles
Dissolve Solute	Add to volumetric flask and dissolve in solvent
Fill to Mark	Add solvent to calibration mark for precise volume
Dilution	Add solvent to lower concentration, using $M_1V_1 = M_2V_2$

Additional info:

• These notes cover foundational concepts in solution chemistry, including molarity, solution preparation, and dilution, which are essential for laboratory and analytical work in general chemistry.

• Further sections (not shown in images) likely cover solution stoichiometry and chemical analysis, as well as introductory thermodynamics.