11 Solubility Factors

8/3/2019 11 Solubility Factors

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Unit 11:

Solution Composition; Energy of Solution; Factors; Vapor Pressure of

Solutions; BPE & FPE; OsmoticPressure; Colligative Properties;



4 different ways to describe concentration

1) Molarity = mol solute / L solvent2) Molality = mol / kg of solvent

3) Mass % = grams solute/ grams solvent

4) Mole fraction = mol solute / mol solvent

Which way of describing solution composition

is dependent upon temperature?



Molarity! Molarity is the only one that

calculates using volume andwhen temperature changes,volume changes.

What will happen to Molarityif temperature rises?

The volume will increase, somolarity will decrease



How can we convert from one toanother?

Dimensional analysis!

Sample problem: #2 in the workbook



Energy of Solution (ΔHsoln) =energy needed for a solute to

dissolve in a solvent

LIKE DISSOLVES LIKE

If temperature drops

when crystals aredissolved in a liquid



Energy of Solution (ΔHsoln) =energy needed for a solute to

dissolve in a solvent

If temperature dropswhen crystals aredissolved in a liquid, is itendothermic or

exothermic?Is ΔHsoln positive ornegative?



Example: Determine whether each of

the following are likely to be water solublea. CH3-CHNH2 d. CH3(CH4)4CH2NH2

OH

H

b. H2C N – C6H5 e. H2C - CH2

NC6H5 OH OH

c. C4H9CH=CH2



Solution:

a. Water soluble: small molecule with 3 polar

bondsb. Insoluble: large molecule with one polar bond

(N, 1 lone pair of electrons)

c. Insoluble: nonpolar no dipole moment

d. Insoluble: large molecule only 1 polar bond (N)

e. Water Soluble : small molecule with 2 polarbonds



Henry’s Law : P=kC

(at constant T) P= partial pressure in atm

C= concentration of dissolved gas (mol/L or M)

k= constant for a solution (L·atm/mol)

Sample problems: 18 and 19



Effects of Pressure

No effect onsolubility of

solids.Solubility of gases

INCREASEas Pincreases.



Effects of Temperature

Solubility of solidsINCREASES as Tincreases (usually).



Effects of Temperature

Solubility of gasesDECREASE as

P increases.(Think of asoda getting

warmer – thegas leaves, andit becomes

flat.)

d dd l ff



How does adding solutes affectvapor pressure?

H d ddi l ff




When the solute isnon-volatile, VP will FALL!

Psoln= xsolvent P˚solvent

(Raoult’s Law)

Psoln=Vapor Pressure of Solution

( atm orTorr)

xsolvent= mole fraction of solvent

P˚solvent= VP of pure solvent

Sam le Problem: #24



Sample Problem: #24

Psoln= xsolvent P˚solvent

Psoln=???

xsolvent= mole solvent/mole total = ???

P˚solvent= 135.3 torr




When it’s volatile, vapor pressure will …?




When it’ s volatile, vapor pressure will INCREASE!

(Raoult’s Law)

Ptotal

=Psolute

+ Psolvent

Psolute = Xsolute P˚solute



What will happen to freezing pointwhen you add a nonvolatile solute?



“Freezing Point Depression”

Freezing point willDECREASE whenyou add anonvolatile solute.



“Freezing Point Depression”

ΔT= kf msolute n

ΔT= change in temperature

kf = molal freezing point constant (this will be given)

msolute= molality of solute (mol solute/kg solvent)

n= number of particles that the solute dissociates into

(example: NaCl dissociates into 2 ions)

Sample Problem: #35

Kf for water = 1.86

Calculate molality by…?

n = 1 (doesn’ t break into ions)



“Boiling Point Elevation”

What willhappen to

boilingpoint whenyou add a

nonvolatilesolute?



“Boiling Point Elevation”

ΔT= kbmsoluten

Same thing,

except now kb= molal boiling point constant

’ t H ff f t (i)



v’ ant Hoff factor (i)Sometimes electrolytes don’ t completely dissociate; ionsstick together.

i = (mols of ions) / (mols solute dissolved)

i depends on the concentration. Ex: At 0.05 m, i = 3.4for FeCl3.

If we increase the

concentration of an

electrolyte solution, i

will ___________.

(increase or decrease?)

’ t H ff f t (i)



v’ ant Hoff factor (i)

If we increase the concentration of an electrolyte

solution, i will decrease.(When there are more ions, they’ re more likely to run into eachother and stick together.)



So, when electrolytes don’ t completely dissociate:

ΔT= (kb or kf )msolutei

(instead of ΔT= kbmsoluten)



Osmosis:Solvent flows through a semipermeable membrane

towards the side with more dissolved solute particles.

O i P d d i



Osmotic Pressure = pressure needed to stop osmosis

j

O i P



Osmotic Pressure = iMRT

= osmotic pressure (atm)M = mol/L

R = gas constant

(.08206 L atm/K mol)

T = temperature (K)

i = v’ ant Hoff factor

What would happen if thepressure on theconcentrated side wasgreater than the osmoticpressure?

R



ReverseOsmosis

By applyingpressure greaterthan osmoticpressure to the

concentratedside, you canforce solventmolecules

through themembrane, andget pure solventon the other

side.



Some Vocab Solution: particles < 1 nm in diameter

Colloid: particles between 1 and 1000 nm Suspension: particles > 1000 nm



Tyndall EffectColloids will scatter light. True solutions won’ t.

If you shine light through a colloid, you can see it from the side.

Sample P oblem



Sample Problem#41: Calculate osmotic pressure of a solution

made by adding 13.65 g sucrose (C12H22O11) toenough water to make 250 mL of solution at 25degrees C.

What equation should I use?

Sample Problem



Sample Problem#41: Calculate osmotic pressure of a solution

made by adding 13.65 g sucrose (C12H22O11) toenough water to make 250 mL of solution at 25degrees C.

= iMRT = osmotic pressure (atm)

M = mol/L

R = gas constant (.08206 L atm/K mol)

T = temperature (K)

i = v’ ant Hoff factor

S l bl



Sample Problem

#46: What is the minimum pressure needed to

desalinate 1.0 M salt (NaCl) solution at 25degrees C?

What equation should I use?

S l P bl



Sample Problem

#46: What is the minimum pressure needed to

desalinate 1.0 M salt (NaCl) solution at 25degrees C?

= iMRT

Follow-up question (which may or may notappear on Friday’ s test):

Will this pressure stay the same as you start topush pure water out of the solution by reverseosmosis? Why or why not?

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11 Solubility Factors

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