Solutions/Concentrations

Georgia Performance Standard SC7: Students will characterize the properties that describe solutions and the

nature of acids and bases.

Review of Aqueous Solutions• Ionic compounds most

readily dissolve in water due to extreme polarity

• Polar covalent compounds also dissolve in water

• Nonpolar compounds don’t

Significant Vocabulary• Solution• Solute and Solvent• Solvation (n); Solvate (v)• Soluble and Insoluble• Solubility• Electrolyte and Nonelectrolyte• Saturate, Unsaturated, and Supersaturated

Determines IF Solute Will Dissolve...

• Like dissolves like: Polar molecules will solvate with polar molecules. Nonpolar molecules will solvate with nonpolar molecules. BUT, polar and nonpolar will not form solutions together.

Determines Speed of Dissolving

1. Agitation: Create more collisions mechanically

2. Temperature: More kinetic energy creates more collisions

3. Surface Area: Dissolving process is a surface phenomenon, the more surface of the solute that is exposed the faster the solvation

BRING SOLUTE IN CONTACT WITH SOLVENT

Determines How Much Will Dissolve

1. Temperature: solubility of solid solute increases as the temp. increases; solubility of gaseous solute decreases as temp increases ex. Hot water bubbles, thermal pollution

2. Pressure: solubility of gaseous solute increases as the pressure increases

-Henry’s Law ex. Soft drinks

SOLUBILITY: HOW MUCH WILL DISSOLVE

Temperature & Solubility• Higher the

temperature, the more solid will dissolve in a liquid

• Higher the temperature, the less gas will dissolve in a liquid

Henry’s Law: Pressure & Solubility

• At a given temperature, the solubility of a gas is proportional to the pressure of the gas above the liquid.

The higher the pressure, the more carbon dioxide will dissolve in the syrup giving a less “flat” taste.

Solubility Graph

Concentration of Solutions

The concentration of a solution is a measure of how much solute is dissolved in a specific amount of solvent or solution.

Molarity: most common units of solution concentration; # of moles solute dissolved in one liter of the solution

Molarity: moles of soluteliter of solution

Example 15.3 (page 531)

Calculate the molarity of a solution prepared by dissolving 11.5 g of solid NaOH in enough water to make 1.50L of solution.

Example 15.4 (page 532)

Calculate the molarity of a solution prepared by dissolving 1.56 g of gaseous HCl into enough water to make 26.8 mL of solution.

molality: moles of solute kg of solvent

If given grams, remember to change it to moles.

The most common concentration term in chemistry is Molarity (M),

but chemists also report concentration in molality (m)

sometimes.

Preparing a Solution in the Lab• You’ve learned about concentration (Molarity and

molality). Therefore, you should be able to create your own solutions for use in the lab from now on.– Example:

• 0.5 M HCl reacts with Mg(s)• If I gave you HCl powder, what would you do to make the

solution?• Watch these kids, and assess your plan.• Choose a lab group and station. Draw an assignment from

the cup, and make the solution. Write your steps (including materials) as you go.

• ALSO: Solve 15-2 Practice Problems WS 6, 8, 10-12, 14-16.

Diluting Solutions

If you already have a solution molarity, but you want a different molarity:Example: Your lab asks you to use 250 mL of 0.25M HCl, but you only find a jug of 6M HCl in the stockroom.

USE THIS EQUATION: M1V1 = M2V2

Example 15.8 (page 539)

What volume of 16 M sulfuric acid must be used to prepare 1.5 L of a 0.10 M H2SO4 solution?

*Be sure to solve the Dilution Worksheet in your practice packet!

Using Molarity as a Conversion Factor

• You have learned to calculate molarity using the equation: – Molarity = moles of solute / liter of solution

• Did you realize that the calculation is simply a ratio of solute to solution?

• Ratios (ie mole ratios, energy to mole ratios) can be used as conversion factors in stoichiometry.

• THEREFORE, molarity can be written into our dimensional analysis charts to solve stoichiometry problems.

Solutions Stoichiometry

• What volume of 1.5 M HCl is needed to react with 21.5 grams of NaOH?

• What is the molarity of a solution of H2S if 48.5mL are required to titrate 35.6mL of 0.35M Fe(OH)3 solution?

• A white precipitate forms when 200 mL of 0.200M K3PO4 solution is mixed with 300 mL of 0.250 M CaCl2 solution. What mass of precipitate will form?

• BE SURE TO SOLVE THE PRACTICE PROBLEMS IN YOUR PACKET!

Physical properties of solution are different from the physical properties of the solvent. Some properties are different simply because there are “foreign” particles (solute) in the solvent. Colligative properties of solutions depend only on the number of solute particles.

Boiling Point Elevation• More solute particles means that vapor pressure

is lower which means that more kinetic energy is needed to make vapor pressure equal atmospheric pressure!

• Change in temperature is calculated:– ∆Tb = Kb m i

• Since boiling point increases...ADD the change.Kb will be given.

Freezing Point Depression

• Liquid particles get into an orderly pattern to become a solid. The solute particles disrupt the orderly pattern causing more kinetic energy to be drawn from the solution for it to freeze!– ∆Tf = Kf m i

• Since freezing point decreases...SUBTRACT the change.

• Kf will be given

Molality

• Another unit for concentration• m=moles of solute per kilogram of solvent• page 520

Solutions Stoichiometry

• A white precipitate forms when 200 mL of 0.200M K3PO4 solution is mixed with 300 mL of 0.250 M CaCl2 solution. What mass of precipitate will form?

• What volume of 1.5 M HCl is needed to react with 21.5 grams of NaOH?

• What is the molarity of a solution of H2S if 48.5mL are required to titrate 35.6mL of 0.35M Fe(OH)3 solution?