SOLUTIONS

TOPICS

Solution Formation

Solubility

Solution concentration

STANDARDS ADDRESSED 6. Solutions are homogeneous mixtures of two or more

substances. As a basis for understanding this concept:

a. Students know the definition of solute and solvent

b. Students know how to describe the dissolving process at the

molecular level by using the concept of random molecular

motion

c. Students know temperature, pressure, and surface area

affect the dissolving process

d. Students know how to calculate the concentration of a solute

in terms of grams per liter, molarity, parts per million, and

percent composition.

OBJECTIVE

By the end of the lesson, the student will: Understand the dissolving process at the molecular level. Understand how the structure of the water molecule

allows it to dissolve many substances Know the factors which influence:

solubility and why dissolving rate and why

Know how to calculate the concentration of a solution

SOLUTION FORMATION

Solution: a homogeneous mixture of two or more substances Solvent: a substance which dissolves another substance

Solute: the dissolved particles; whatever is being dissolved

Solution Formation Vocab:

Soluble: a substance that dissolves in a solvent

Insoluble: a substance that does not dissolve in a solvent

Examples of Solutions

Gas in Liquid: Carbonic Acid (carbonation): CO2 in H2O

Liquid in Liquid Vinegar: Acetic Acid in water

Solid in Solid Brass: copper and zinc

Animation: Dissolving Process

http://www.youtube.com/watch?v=EBfGcTAJF4o (53 sec)

http://www.youtube.com/watch?v=CLHP4r0E7hg&feature=related (42 sec)

http://www.youtube.com/watch?v=xdedxfhcpWo&feature=related (1:35)

The dissolving process at the molecular level:

Let’s look at the structure of the water molecule

The structure of the water molecule creates a polar molecule due to the differences in electronegativities of the oxygen and hydrogen atoms.

Partial negative charge on the oxygen attracts the partial positive charge on the neighboring molecule. Molecules arrange themselves so as to have opposite charges aligned, pulling it away from its ‘structure’, “one ion or molecule at a time”

Dissolving Process

DISSOLVING PROCESS:

Once released, the ions are surrounded by water molecules.

The dissolving process is reversible

The dissolved solute moves around in the solution and when it

comes into contact with un-dissolved solute particles it re-

crystallizes, meaning it returns to the solid state. And the

process repeats itself.

NaCl Na+(aq) + Cl- (aq)

Saturation

When the rates of the dissolving and

recrystallization become the same, the

solution is saturated at that temperature. A

dynamic equilibrium is reached,

meaning the rate of the forward reaction is the

same as the reverse reaction.

FACTORS AFFECTING DISSOLVING RATE

Where does dissolving occur?

On the surface of a substance To increase dissolving rate, contact between

solvent and solute must increase 3 common ways to increase the collisions between solute and

solvent Temperature Amount of Surface Area Exposed Agitation

Temperature

Temperature: raising the temperature increases the kinetic energy of the particles, resulting in more frequent and forceful collisions.

Amount of Surface Area Exposed

Amount of Surface Area Exposed: breaking the solute into smaller pieces increases its surface area. A greater surface area allows more collisions to occur and therefore, faster dissolving

Agitation (mixing, stirring, etc.)

Agitation: stirring moves dissolved solute particles away from the contact surfaces more quickly and thereby allows new collisions between solute and solvent particles to occur. Without stirring, solvated particles move away from the contact areas slowly.

SOLUBILITY the maximum amount of solute that will dissolve in a

given amount of solvent at a specified temperature and pressure

3 Types of Solutions

Saturated solution

Unsaturated solution

Supersaturated solution

Saturated solution: contains the maximum amount of solute. You cannot dissolve any more solute in the solvent.

Unsaturated solution: contains less than the maximum amount of solute. You can dissolve more solute in the solvent.

Supersaturated solution

contains more than the normal maximum amount of solute. This is usually achieved by heating the solution in order to dissolve more solute, then the solution is cooled. This makes a supersaturated solution. (ie.Rock candy; hot mineral springs)

Supersaturated Solutions

STALAGMITES ROCK CANDY

3 Factors Affecting Solubility 1. Temperature:

many substances are more soluble at high temperatures than low temperatures. Exception: Gases

Solubility of Gases with Increased Temperature

As Temperature increases, Solubility of gases decreases

Pressure affect on gases

2. Pressure: affects the solubility of

gaseous solutes. The solubility of a gas in any solvent increases as the pressure above the solution increases

3 Factors Affecting Solubility

2. Pressure: affects the solubility of gaseous solutes. The solubility of a gas in any solvent increases as the pressure above the solution increases

3 Factors Affecting Solubility 2. Pressure: affects

the solubility of gaseous solutes. The solubility of a gas in any solvent increases as the pressure above the solution increases

3 Factors Affecting Solubility 3. Intermolecular forces: “like dissolves like;”

meaning polar substances dissolve polar substances and non-polar substances dissolve non-polar substances.

Miscibility: the ability to mix without separating into two phases

Miscible: substances that mix togetherie. Vinegar and water

Immiscible: substances that do not mix together

ie. Oil and water

SOLUTION CONCENTRATION: the amount of solute in a solution

Dilute solution: contains small amounts of solute (dissolved particles)

Concentrated solution: contains large amounts of solute

MASS PERCENT:the number of grams of solute per 100 grams of solution

Mass Percent = mass of solute x 100%

mass of solution

EXAMPLE 1

Calculate the mass percent of NaCl in a solution containing 15.3 g of NaCl and 155.0 g water. (8.98%NaCl)

EXAMPLE 2

Calculate the mass percent of a solution containing 27.5 g of ethanol and 175 mL of water. The density of water is 1.0 g/mL. 13.6%)

Example 3

You have 1,500 g of a bleach solution. The percent by mass of the solute sodium hypochlorite, NaOCl, is 3.62%. How many grams of NaOCl are in the solution? (54.3 g)

MOLARITY (M): the number of moles of solute dissolved per liter of solution.

Note: Here, volume refers to the total volume of the solution,not the volume of the solvent.

Molarity (M) = moles of solute

liters of solution

M is pronounced “molar”

EXAMPLE 1

Calculate the molarity if water is added to 2.0 mol of

glucose to give 5.0 L of solution. (0.40 M)

EXAMPLE 2 A solution contains 0.900 g of NaCl per 100 mL of solution.

What is the molarity? (0.154 M)

Example 3

How many moles of Na2SO4 are needed to prepare 1.5L of 0.20

M Na2SO4? (0.30 moles solute)

Example 4

How many grams of solute are contained in 250 mL of 3.0

M NaOH? (30. g NaOH)

Example 5

How would you prepare 250 mL of a 0.500 M NaCl solution? (7.31 g NaCl)

Preparing a Solution from a Solid

SOLUTION DILUTION:

adding solvent to a concentrated (stock) solution to make a less concentrated (dilute)solution

DILUTION

Moles Before= Moles After

Number moles before dilution = number moles after

dilution

Use the dilution equation

M1 x V1 = M2 x V2

QUESTION 1

How many milliliters of 12.0 M HCl are needed

to prepare 750. ml of 0.250 M HCl? (15.6 mL)

QUESTION 2

How would you prepare 5.00 L of a 1.50 M

KCl from a 12.0 M stock solution? (0.625 L)

Making a dilute solution from a concentrated solution

Example 3 What is the molarity of the resulting solution when

the following mixture is prepared? 150.0 mL of water is added to 55.0 mL of 6.50 M NaOH. (1.74 M)

Example 4 How much water must be added to 125 mL of a

4.50 M NaCl solution to produce a 2.75 M solution? (80. mL)

SUMMARY OF STEPS

1. Construct a chart of values

2. Rearrange equation so that unknown is

isolated

3. Convert grams to moles, if necessary

4. Calculate and report to correct # of sig. figs!

SOLUTION STOICHIOMETRY

There are several different ways to solve solution stoichiometry problems. Here are a few guidelines/suggestions to get you started:

Write a balanced chemical equation use Molarity as mol…

L

***allows conversion between moles and volume or concentration and volume

use mol-mol ratio if necessary there may be limiting reagent problems…in this

case determine how many moles there are for each substance

sample mini road map:

mL→L→mol→mol→g

L

…from moles you can go to L (volume) or grams (mass)…or even particles via Avogadro’s #!

Example 1 1) Calculate the mass of solid NaCl that must be added

to 1.50 L of a 0.100 M AgNO3 solution to precipitate all the Ag+ ions in the form of AgCl

Example 2 2) How many milliliters of 0.0500M Pb(NO3)2 are

needed to react with 2.00L of a 0.0250M Na2SO4 solution in order to produce a precipitate? How many grams of precipitate are formed?

END