I. The Nature of Solutions

SOLUTIONS “If you’re not part of the solution, you’re part of the

precipitate”

Properties of Solutions

Definition of a Solution: a homogeneous

mixture of substances in the same physical

state

What does Homogeneous mean?

atoms, ions, or molecules of one

substance spread uniformly throughout

a second substance

Types of Solutions Solutions exist in all three states (solid, liquid

and gas).

What is it called when metals are mixed to form a solid solution? alloy

ex. brass = Cu/Zn; steel = iron + carbon

What is an example of gases forming a solution? Air (is a homogeneous mixture)

The majority of this topic will be limited to only liquid solutions, in which a solid is dissolved in a liquid.

• ex. salt water, antifreeze

1. Solute substance being dissolved; smaller amount.

2. Solvent substance doing the dissolving; larger amount

Salt in salt water Sugar in soda drinks

Carbon dioxide in soda drinks

Water in salt water Water in soda

What is the most common solvent? Water What are these types of solutions called Aqueous

solutions and what is the notation used (aq)?

What makes a SOLUTION

Concentrated vs. Dilute

Write an example equation of a substance being dissolved in water:

Dissociation - ionic solid separating into ions

NaCl(s) Na+(aq) + Cl

–(aq)

hydrated ions

LiCl(s) Li+(aq) + Cl–(aq)

5 Main points about all

liquid solutions…..

1.Solutions are homogeneous mixtures (aq) –known as true solutions

2.Solutions are clear & do not disperse light

3.Solutions can have color

4.Solutions will not settle upon standing

5.Solutions will pass through a filter

Solubility Factors

Define Solubility: amount of solute that will dissolve in a certain amount of a solvent at a certain temperature

Materials with high solubility are said to be SOLUBLE (aq)

Materials with low solubility are said to be INSOLUBLE (s) –precipitate out

Use Table F (Solubility Guidelines) – see pg 4 of guide for practice

THE NATURE OF SOLUTE AND SOLVENT

Draw the dissolving process of NaCl in water - dissociation

• The positive ends of the Na+ are attracted to the NEGATIVE end of the water molecule.

• The negatively ends of the Cl- are attracted to the POSITIVE end of the water molecule.

• Opposites ends attract

NaCl (s) Na+ (aq) + Cl

– (aq)

• This dissolving process of a solid in water is called HYDRATION or

SOLVATION

• What does the phrase “Like dissolves like ‘ mean? DESCRIBES WHAT

SOLUTES WILL DISSOLVE IN WHAT SOLVENTS

Ex of Ionic Solvation Ex of Molecular (covalent) Solvation

(molecules stay intact)

NaCl (s) Na+ (aq) + Cl–

(aq) C6H12O6 (s) C6H12O6 (aq)

Solvation and Conductivity

Strong

Electrolyte

Non-

Electrolyte

solute exists as

ions only

- +

salt

- +

sugar

solute exists as

molecules only

- +

acetic acid

Weak

Electrolyte

solute exists as

ions and

Molecules

Salts dissociate

Covalent (nm + nm)

Includes acids

Ionic (m + nm)

Electrolyte = conductor

Fill in the Solubility Summary

based on similar attractive forces

“Like Dissolves Like”

Solute type nonpolar solvent

(soap)

polar solvent

(water)

Nonpolar (oil)

soluble Insoluble

Polar (sugar)

insoluble soluble

Ionic (salt)

insoluble soluble

Variables: see table G

1. Temperature

As temperature increases, most solids become more soluble in water.

As temperature increases, most gases become less soluble in water.

• Think warm soda pop

• solids are more soluble at high temps.

2. Pressure

Pressure has little or no effect on the solubility of solid or liquid solutes.

Pressure does affect the solubility of gases in liquids. As pressure Increases, the solubility of gases in a liquid increases.

gases are more soluble at low temps & high pressures

Graphs – Table G

pressure

Sol

(g)

Sol

(L & S) Sol

(g)

Temp temp

inc P, inc sol inc T, dec sol inc T, inc sol

A cube of sugar

in cold tea

dissolves

slowly. .

Granulated sugar

dissolves in cold

water more quickly

than a sugar cube,

especially with stirring

Solution Formation

Granulated

sugar dissolves

very quickly in

hot tea

RATE OF SOLUTION

Factor

Affect on Solid Solute

Affect on Gaseous Solute In a confined space

Crushing

reducing particle size increases the rate by increasing surface area (causing more collisions)

not applicable

Stirring

increases the rate by exposing fresh solvent to solute and increasing kinetic energy (motion of particles)

decreases the rate by increasing kinetic energy, thereby reducing solubility

Amount of dissolved solute

as the amount of dissolved solute increases, the rate decreases

as the amount of dissolved solute increases, the rate decreases

Temperature

as the temp increases, the rate increases due to increasing kinetic energy causing molecules to collide more frequently

as the temperature increases, the rate decreases solubility

Shows the relationship between temp and solubility of a solute

Table G (based on 100g of water)

Solids increase temp, increase solubility Solid line

Gas: increase temp, decrease solubility Dotted line

Saturation & Solubility Curves

Saturation

•A solution that contains the maximum amount of solute in a given amount of solvent. •On the reference line

the rate of dissolving equals the rate of crystallization (equilibrium), so the total amount of dissolved solute remains constant

NaCl(s) NaCl(aq)

Implies equilibrium

Saturated Solutions

dissolving

crystallization

Unsaturated

A solution that

contains less

solute than a

saturated solution

under existing

conditions is

unsaturated.

Below the

reference line

Supersaturated

A supersaturated solution is clear before a seed crystal is added.

Crystals begin to form in the solution immediately after the addition of a seed crystal.

Supersaturated

Excess solute crystallizes rapidly

Practice Question #1

1) According to Reference Table G, which compound solubility decreases most rapidly as the temperature changes from 10°C to 70°C?

a) NH4Cl

b) NH3

c) HCl

d) KCl

Answer to problem #1

Correct Answer Number: 2

Explanation: See Ref. Table G.

Notice the curves for choices 1 and

4 increase. Choice 2 and 3 both

decrease but choice 2 (NH3)

decreases more than 50 degrees,

while HCl decreases only about 16

degrees.

Molarity

• The concentration of a solution is a measure of the amount of solute that is dissolved in a given quantity of solvent.

– A dilute solution is one that contains a small amount of solute.

– A concentrated solution contains a large amount of solute.

Concentrations of Solutions

• Water must be tested continually to ensure that the concentrations of contaminants do not exceed established limits. These contaminants include metals, pesticides, bacteria, and even the by-products of water treatment. You will learn how solution concentrations are calculated.

Quantities in Solutions

The amount of solute in a solution.

Describing Concentration

• % by mass - medicated creams

• % by volume - rubbing alcohol

• ppm, ppb - water contaminants

• molarity - used by chemists

• molality - used by chemists

Molarity

• Molarity (M) is the number of

moles of solute dissolved in one

liter of solution.

• To calculate the molarity of a solution,

divide the moles of solute by the volume of

the solution.

Molarity

• To make a 0.25 molar (0.25M) solution, first

add ? mol of solute to a 0.50-L (500mL)

volumetric flask half filled with distilled water.

Solve for Moles

Solute used is NaCl

We want to prepare 500 mL of a 0.25

Molar solution

Using the Molarity formula solve for

moles of solute

M = Moles of Solute

Liters of Soln

Plug in Numbers

0.250 mol/Liter = x moles

0.500L

# of Moles = 0.250mol/liter x 0.500 L

x = 0.125 moles

Convert Moles to Grams

0.125 moles NaCl ? Grams

Use Mole Conversion Formula

Moles = given mass

gfm

Plug in Numbers

0.125 moles = x grams

58 g/mol (gfm of NaCl)

X= 7.25 grams NaCl

Molarity

• Add 7.25 grams of NaCl to water in flask

• Swirl the flask carefully to dissolve the solute.

Molarity

• Fill the flask with water exactly to

the 500-mL mark.

16.2

#1: How many moles of NaOH are contained in 200 ml of 0.1M solution of NaOH?

2) Which solution is the most concentrated?

a) 1 mole of solute dissolved in 1 liter of solution?

b) 2 moles of solute dissolved in 3 liters of solution?

c) 6 moles of solute dissolved in 4 liters of solution?

d) 4 moles of solute dissolved in 8 liters of solution?

Answer:

moles = (M)(L)

= (0.1 M)(.2L)

=.02 mol

Correct Answer Number: 3 Explanation: #3 (1.5 M. ) is the most concentrated. Find the molarity ( moles of solute/liter of solution) for

each answer.

#1) 1 mole / 1 liter or 1 M. #2) 2 moles / 3 liters or 0.67 M. #3) 6 moles / 4 liters or 1.5 M. #4) 4 moles / 8 liters or 0.5 M.

Answer:

M = (moles)/(L)

= 1.50 moles

.5000L

= 3.00 M

Ex #2: What is the molarity of a 500.0 mL solution of NaOH (GFM = 40.0g) with 60.0 g of NaOH (aq)?

Different style problem

1) Which solution is the most concentrated?

a) 1 mole of solute dissolved in 1 liter of solution?

b) 2 moles of solute dissolved in 3 liters of solution?

c) 6 moles of solute dissolved in 4 liters of solution?

d) 4 moles of solute dissolved in 8 liters of solution?

Answer to different style problem

Correct Answer Number: 3 Explanation: #3 (1.5 M. ) is the most concentrated. Find the molarity ( moles of solute/liter of solution) for

each answer.

#1) 1 mole / 1 liter or 1 M. #2) 2 moles / 3 liters or 0.67 M. #3) 6 moles / 4 liters or 1.5 M. #4) 4 moles / 8 liters or 0.5 M.

Percent Solutions

• Concentration in Percent

(Volume/Volume)

16.2

Percent Solutions

• Isopropyl alcohol (2-propanol) is sold as a 91%

solution. This solution consist of 91 mL of

isopropyl alcohol mixed with enough water to

make 100 mL of solution.

16.2

Percent Solutions

Percent Solutions

Percent Solutions

• Concentration in Percent (Mass/Mass)

16.2

A 50.0 gram sample of a solution is evaporated and found to contain 0.100 grams of sodium chloride. What is the percent by mass of sodium chloride in the solution?

% mass = 0.100 g X 100

50.0 g

= 0.200%

100.0 grams of water is evaporated and analyzed for lead. 0.00010 grams of lead ions are found. What is the concentration of the lead, in parts per million?

ppm = grams of solute x 1,000,000 grams of solution = (0.00010 g) x 1,000,000 100.00010 g = .9 ppm •If the legal limit for lead in the water is 3.0 ppm, then the water sample is within the legal limits (it’s safe and ok)

Colligative Properties of

Solutions

The wood frog is a remarkable creature because it

can survive being frozen. Scientists believe that a

substance in the cells of this frog acts as a natural

antifreeze, which prevents the cells from freezing.

You will discover how a solute can change the

freezing point of a solution.

16.3

Colligative Properties

depend on the number of particles (molality) rather than the nature of the particles in the solution.

Boiling point, freezing point, vapor pressure and osmotic pressure are some of the properties affected.

Boiling Point Elevation

BP of a solution is higher than BP of the pure solvent

Solute particles weaken IMF in the solvent

Freezing Point Depression

• f.p. of a solution is lower than f.p. of the

pure solvent

Vapor-Pressure Lowering

• In a solution, solute particles reduce the number

of free solvent particles able to escape the liquid.

Equilibrium is established at a lower vapor

pressure.

16.3

Vapor-Pressure Lowering

Three moles of

glucose dissolved in

water produce 3 mol

of particles because

glucose does not

dissociate.

16.3

Three moles of

sodium chloride

dissolved in water

produce 6 mol of

particles because

each formula unit of

NaCl dissociates into

two ions.

Three moles of calcium

chloride dissolved in

water produce 9 mol of

particles because each

formula unit of CaCl2

dissociates into three

ions.

Applications

• salting icy roads

• making ice cream

• antifreeze

•cars (-64°C to 136°C)

•fish & insects

Colligative Properties con’t.

• # of Particles

• Nonelectrolytes (covalent) – remain intact when dissolved – 1 particle

• Electrolytes (ionic) – dissociate into ions when dissolved – 2 or more particles

Boiling point elevation

called molal boiling point elevation (Kb).

It is proportional to the concentration of dissolved particles

1 mole of particles in solution raises the BP of water by 0.52 C

Which 1 molal solution will have the highest boiling point?

KNO3 Mg(NO3)2 Al(NO3)3

Freezing Point

Depression

called molal freezing point depression (Kf).

1 mole of particles in solution depresses the FP of water by 1.86 C

What will depress the FP of water the most….AlCl3or MgCl2? Why? What would the new FP be?