Solutions and Acids and Bases - Lesson 2: The Solution Process and Concentration

Unit 9 – Solutions and Acids and Bases Lesson 2. The Solution Process and Concentration

Big Idea:

• Solutions are homogeneous mixtures of a solute in a solvent.

• Chemical and physical factors affect how much of one substance will dissolve in another.

Learning Objective:

• Identify the physical and chemical factors that affect the solubility and rates of dissolution, such as temperature and surface area.

• Calculate concentration of solution (% mass and % volume)

Main Ideas:

• Several factors affect dissolving.

• Solubility is a measure of how well one substance dissolves in another.

• A change in energy accompanies solution formation.

Key Terms:

• solution

• equilibrium

• % by mass

• solubility

• % by volume

• hydration

• unsaturated solution

• saturated solution

• supersaturated solution

• immiscible

• miscible

Miscible vs Immiscible Liquids:

• Miscible: Liquids that can dissolve in one another.

• Immiscible: Liquids that do not dissolve in one another.

Components of a Solution:

• Solute: The substance dissolved in a solution, usually present in a lesser amount.

• Solvent: The dissolving medium in a solution, usually present in a greater amount.

Hydration:

• The solution process with water as the solvent is referred to as hydration.

• Ions are said to be hydrated; for example, $Li^+(aq)$, $Cl^-(aq)$, where 'aq' stands for aqueous.

Dissolving Ionic Compounds in Aqueous Solution:

• Hydrated copper(II) sulfate ($CuSO<em>4 cdot 5H</em>2O$) has water as part of its crystal structure.

• Heating releases the water and produces the anhydrous form of the substance, which has the formula $CuSO_4$.

Factors Affecting Dissolving:

Increasing the Surface Area of Solute:

• The rate at which a solid solute dissolves can be increased by increasing the surface area.

• A powdered solute has a greater surface area exposed to solvent particles and therefore dissolves faster than a solute in large crystals.

Stirring or Agitating a Solution:

• Stirring speeds up dissolving because fresh solvent is continually brought into contact with the surface of the solute.

Heating a Solvent

Types of Saturation:

• Unsaturated Solution: More solute dissolves.

• Saturated Solution: No more solute dissolves.

• Supersaturated Solution: Becomes unstable, crystals form.

• A solution that contains the maximum amount of dissolved solute is described as a saturated solution.

• A solution that contains less solute than a saturated solution under the existing conditions is an unsaturated solution.

• A supersaturated solution is a solution that contains more dissolved solute than a saturated solution contains under the same conditions (25°C).

Saturation Point:

• The graph shows the range of solute masses that will produce an unsaturated solution.

• Once the saturation point is exceeded, the system will contain undissolved solute.

• Unsaturated: If a solution is unsaturated, more solute can dissolve. No undissolved solute remains.

• Saturated: If the amount of solute added exceeds the solubility, some solute remains undissolved.

• Solubility = 46.4 g/100 g

Solution Equilibrium:

• Solution equilibrium is the physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates.

• Rate of dissolution = rate of crystallization in a saturated solution.

Solubility:

• Solubility is the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature.

Concentration of Solutions:

• The concentration of a solution is a measure of the amount of solute in a given amount of solvent or solution.

Percent Volume:

Percent Mass:

• $$

• Solution = solvent + solute

Percent Volume Examples:

• Example 1: What is the percent by volume concentration of a solution in which 75.0 mL of ethanol is diluted to a volume of 250.0 mL?

  • $$

• Example 2: What volume of acetic acid is present in a bottle containing 350.0 mL of a solution which measures 5.00% concentration?

  • $x = 0.05 \times 350.0 \space mL = 17.5 \space mL$

Percent Mass Example:

• Example 3: Find the percent by mass in which 41.0 g of NaCl is dissolved in 331 grams of water.

  • Mass solution = 41.0 g + 331.0 g = 372.0 g

  • $$

Quick Lab Activity: Percentage by Volume and Percentage by Mass:

• This activity demonstrates how to calculate percentage by volume and percentage by mass using simple mixtures.

• Materials:

  • 10.0 mL of Alcohol (e.g., ethanol)

  • Beakers

  • 50.0 mL of Water

  • Graduated cylinders

  • 10.0 grams of Sodium Chloride (NaCl)

  • Balance scale (for mass)

  • 50.0 mL of Water

Part 1: Percentage by Volume (Alcohol in Water)

1. Measure 10.0 mL of alcohol using the graduated cylinder.

2. Measure 50.0 mL of water using the graduated cylinder as well.

3. Mix the alcohol with the water in a clean beaker.

4. Calculate the total volume of the mixture.

5. Calculate the percentage by volume of alcohol.

Part 2: Percentage by Mass (NaCl in Water)

1. Weigh 10.0 grams of NaCl using the balance scale.

2. Measure the mass of 50.0 mL of water on the balance scale.

3. Mix the NaCl into the water and stir until it is dissolved.

4. Calculate the total mass of the solution.

5. Calculate the percentage by mass of NaCl.