SolubilityUnit IIILesson 1

Unit Intro • Our focus is on solutions of aqueous ions• As you know; acids, bases and salts form

ionic solutions. • This unit is only concerned with salts.

ReviewElectrolytes: substances that dissolve to give electrically conducting solutions that contain ions.

Ex:

Review Non-electrolytes: a substance that dissolves to give non-conducting solutions containing only neutral molecules.

Ex:

Molecular vs Ionic solutionsIonic solutions

metals and non-metals

compounds that contain polyatomic ions

Molecular compounds They are covalent compounds

Non-metal and non-metalespecially organic compounds

Useful hintWhen it comes to our chem12 course, you are only going to deal with ionic compounds that have one type of positive ion and one type of negative ion only.

 Ionic Solutions Molecular/Covalent Solutions NaCl(aq) C6H12O6(aq) metal or polyatomic ion Ca(OH)2(aq) C12H22O11(aq) nonmetal or carbon (NH4)3PO4(aq) CH3OH(aq)

 Ca(CH3COO)2(aq) O2(aq)

 H2SO4(aq) N2H4(aq)

 Conduct electricity Do not conduct electricity

Write equations to show the dissolving of the following substances in water

NaCl(s) Na+ + Cl-

C6H12O6(s) C6H12O6(aq)

Ca(OH)2(s) Ca2+ + 2OH-

C12H22O11(s) C12H22O11(aq)

(NH4)3PO4(s) 3NH4+ + PO4

3-

CH3OH(l) CH3OH(aq)

Back to solubility In chem 11, you learnt that…

The solubility of a substance is the maximum amount of the substance which can dissolve in a given amount of solvent at a given temperature.

Equilibrium Solubility The solubility of a substance is the equilibrium concentration of the substance in solution at a given temperature.

when expressed in moles/L it’s called Molar Solubility.

Solubility at equilibirum

Solid MgCl2 dissolves and enters solutiondissolving reaction

Mg+2 + Cl-1 ions come together to form MgCl2

crystallization reaction

Solubility at equilibriumWhen the rate of dissolving reaction equals the rate of crystallization reaction, we have equilibrium.

– A solution at equilibrium is called a saturated solution.

Saturation exists when…• Equilibrium exists between the dissolved

(ions) and the undissolved material (solid)

• Some undissolved material is till present (crystal solids)

How to saturate a solution & determine solubility

• To saturate a solution, add weighed portions of your solid to a volume of (solvent) water and stir until full. – A bit of excess solid will always be present at

equilibrium saturation though.

• In order to determine the solubility, you must completely fill or saturate the solution!

Mg2+ Cl-

Amount MgCl2 DissolvedRate of dissolving > Rate of crystallization

Determining The Solubility of MgCl2

Add measured portions of MgCl2 to 100.0 mL and stir to dissolve

MgCl2

100.0 mL

10.0 gMgCl2 10.0 gMgCl2

10.0 g

MgCl2

3.0 g

MgCl2

0.0 g

Rate of dissolving = Rate of crystallization

slowvery slow

33.0 g equilibrium

unsaturated

saturated

MgCl2(s)

Calculate the solubility in units of g/L and mole/L

= 3.46 M

0.100 L95.3g

x 1 mole33.0 g

=

Molar Solubility = Moles/L

Solubility = 33.0 g0.100 L

= 330. g/L

Equilibrium Equation MgCl2(s) ⇌ Mg2+ + 2Cl-

Expression: Keq = [Mg2+][Cl-]2

do not use the solid

The Ksp or (solubility product) is used for saturated solutions at equilibrium

Ksp = [Mg2+][Cl-]2

Un-Saturated, Saturated and Super-satured Review

The rate of dissolving > the rate of crystallizing

Not at equilibrium

Not full – ( more solid can dissolve if you add it)

Unsaturated Solutions

How does it look? Clear solution!

The rate of dissolving = the rate of crystallizing

At equilibrium

Full- ( adding more solid will not dissolve )

How does it look? it always has crystals/solids in the solution.

Saturated Solutions

The rate of dissolving < the rate of crystallizing

Not at equilibrium

Over full – ( adding more solid causes precipitation)How does it look? Clear solution!

Supersaturated Solutions

Supersaturated video

Predicting the solubility of salts Page 332 in your textbook has the table “ Solubility of common compounds in water”

Will be provided for youUse it to predict solubility ( high or low )Use it to predict if a precipitate will form.

Ag2SO4

CuSO4

CuCl2

CuCl

BaS

K2CO3  

FeSO4

CaSO4

Ca(NO3)2

Na3PO4 High

Low

High Solubility means > .1 MLow Solubility means .1M

High

High

High

High

Low

High

Low

High

Precipitate Questions Will a precipitate form when 0.2 M solutions of CaS and Na2SO4 are mixed?

Write the equation for equilibrium present in a saturated solution of Al2(SO4)3(s)

solution.

Ksp = [Al3+]2[SO42-]3

Equilibrium Expression

Al2(SO4)3(s) ⇌ 2Al3+ + 3SO42-

Write the equation for equilibrium present in a saturated solution of Ca3(PO4)2(s) solution.

Ksp = [Ca2+]3[PO43-]2

Ca3(PO4)2(s) ⇌ 3Ca2+ + 2PO43-

Equilibrium Expression

pg 74 #1-2, pg 76 #3-7, pg 77 #8-11Pg 83 #21,22 Pg 84 #24.

Practice Time