Molarity and - PowerPoint Presentation

Slide1

Molarity and Colligative Properties

Chemistry GT 5/8/15Slide2

Drill

Use the table from pg.

9

to give the amount of substances that will dissolve in 100

g

of H

2

O:

NH

3

at 10°C and 80°C

Why does solubility of NH

3

decrease at higher temperatures?

KCl

at 10°C and 70°C

If 50

g

of

KCl

were dissolved in 100

g

of water, and it was cooled to 50°C, what kind of solution would you have?

HW:

pg. 5

Molality

Problems

Effect of a Solute on FP and BPSlide3

Objectives

Today I will be able to:

Calculate the

molality

of a solution

Describe

the 4

colligative

properties of vapor pressure, boiling point, freezing point and osmotic pressure

Calculate the

Van’t

Hoff Factor for a Compound

Calculate the freezing point depression and boiling point elevation of a soluteSlide4

Agenda

Drill

Molality

Notes & Example

Colligative Properties Research Chart

Colligative Properties Notes

Colligative Properties Calculations

Exit TicketSlide5

Molality

Don’t you mean

molarity??

Nope!

Molality

is another way to measure concentration.

Equation:

m

=

n

m

n

= moles of solute

m

= mass of solvent in

kilogramsSlide6

Why??

Molarity changes with the density of the solvent, so at different temperatures, you have different molarities.

Molality

does NOT change with density or temperature!

Let’s do #1 on pg. 5Slide7

colligative properties!

What’s next?Slide8

Colligative Properties Chart

You have 15 minutes to fill in the chart of notes.

Split the work up at your table—each person can take one quadrant, and then share.

You may use a textbook or BYOD

We

will review the chart togetherSlide9

Colligative Properties NotesSlide10

Colligative Properties

A property that depends on the number of molecules present, but not on their chemical natureSlide11

There are 4 colligative properties

Vapor Pressure

Boiling Point

Freezing Point

Osmotic PressureSlide12

Vapor Pressure

Liquid molecules at the surface of a liquid can escape to the gas phase

- This process is reversible (g

 l)

Vapor pressure of a solution containing a nonvolatile solute is less than the pressure of the solvent aloneSlide13

Vapor Pressure

Solute particles take up surface area and lower the vapor pressureSlide14

Vapor Pressure

Vapor pressure reduction is proportional to the concentration of the solution

When the concentration goes up, the vapor pressure is reducedSlide15

Vapor Pressure

This partially explains why The Great Salt Lake has a lower evaporation rate than expected. The salt concentration is so high that the vapor pressure (and evaporation) has been lowered

Slide16

Freezing Point

The freezing point of a solution is always lower than that of the solvent alone

This is called Freezing Point Depression

Explains why salt (CaCl

2

) is used on ice

- The salt solution that forms has a lower freezing point than the original iceSlide17

Boiling Point

The boiling point of a solution is always higher than that of the solvent alone

Boiling Point Elevation

You continue to use antifreeze in the summer, because you want the coolant to boil at a higher temperature so it will absorb the engine heatSlide18

Osmotic Pressure

The tendency for a solution to take in water due to osmosis

This is why cells have to maintain their internal pressures—if they stop pumping water out, they will fill up and pop!Slide19

Boiling and Freezing Point CalculationsSlide20

Before we calculate… we need to talk about the Van’t Hoff FactorSlide21

Van’t Hoff Factor

Determines the moles of

particles that

are present when a compound dissolves in a solution

Covalent compounds do not dissociate

C

12

H

22

O

11

1 mole

(

Van’t

Hoff Factor = 1, the same

for all

nonelectrolytes

)

Ionic Compounds can dissociate

NaCl



Na

+

+

Cl

-

2 moles of ions (

Van’t

Hoff Factor = 2)

CaCl

2



Ca

+2

+ 2

Cl

-

3 moles of ions (

Van’t

Hoff Factor = 3)Slide22

Determine the Van’t Hoff Factor for the following Compounds

C

6

H

12

O

6

KCl

Al

2

O

3

P

2

O

5Slide23

Calculating Boiling and Freezing Points



T

b

= K

b



m



i

K

b

is the

molal

boiling point elevation constant, which is a property of the

solvent

K

b

(H

2

O) = 0.52°C/m

m

is

molality

i

is the

Van’t

Hoff Factor



T

b

is added to the normal boiling pointSlide24

Calculating Boiling and Freezing Points



T

f

=

K

f



m



i

K

f

is the

molal

freezing point depression constant, which is a property of the

solvent

K

f

(H

2

O) = 1.86°C/m

m

is

molality

i

is the

Van’t

Hoff

Factor



T

f

is subtracted from the normal freezing pointSlide25

Boiling and Freezing Points and Electrolytes

What is the expected change in the freezing point of water in a solution of 62.5 grams of barium nitrate, Ba(NO

3

)

2

, in 1.00 kg of water?

∆

T

f

=

K

f



m



i

62.5

g

Ba(NO

3

)

2



.239 moles

.239 moles/1.00 kg = .239

m

1.86°C/m

x

.239

m

= .444°C

Ba(NO

3

)

2



Ba

+2

+ 2 NO

3

-1

= 3 moles of ions (

i

value)

.444°C

x

3 = 1.33°C

0°C – 1.33°C = -1.33°CSlide26

Exit Ticket

Determine the

Van’t

Hoff Factor for the following compounds.

AlCl

3

Mg

3

(PO

4

)

2

C

6

H

12

O

6