Solubility
Definitions:
•
Electrolyte — dissolves to give an electrically conducting solution (ions present in solution)
— ionic solutions.
•
Non-electrolyte — dissolves to give a non-conducting solution (no ions in solution, only
neutral molecules) — molecular solutions.
Ionic Solutions form from ionic compounds — metal and non-metal or a polyatomic ion.
Molecular solutions form from covalent compounds — two non-metals
Note: OH as an alcohol, and OH– as hydroxide.
Example 1: Write the reactions for the following dissolving in water.
a) NaCl
b) Sr(OH)2
c) (NH4)2SO4
d) CH3OH
e) C6H12O6
Solubility —
Saturated solution is a solution
Solubility is the
Molar Solubility is expressed in mol/L (M)
A solution is saturated if:
a) Some undissolved material is present (even only microscopic amounts),
and,
b) There is an equilibrium between the dissolved and undissolved materials.
AgCl(s)  Ag+(aq) + Cl–(aq)
Example 2:
2.15 g of CaCO3 will dissolve in 500.0 mL of water. What is the molar solubility of CaCO3?
Example 3:
The solubility of Silver chloride is
of water?
7.3 x 10-5 M. How many grams will dissolve in 10.0 L
Example 4:
2.4 g of CaI2 is dissolved in 255 mL of water. What are the concentrations of all ions in this solution?
pp. 74-81: 1, 2, 3-17(odd), 18
II. Solubility — Precipitates
To determine if an ionic compound is soluble, we are given an experimentally determined chart
of ionic combinations.
See “Solubility of Common Compounds in Water” chart in your data booklet.
Notes on Solubility Table:
•
•
•
•
•
Temperature is 25°C
Soluble is defined as dissolving more than 0.1 M
A low solubility combination will form a precipitate.
The copper ion that precipitates with halides is Cu+ (copper I)
Alkali ions includes Li+ to Fr+.
Example 1: Which salts are soluble?
a) BaSO4
b) CaS
c) CuCl2
d) K3PO4
e) Al(OH)3
f) MgSO3
g) CuCl
Example 2: Will the following combinations of solutions form a precipitate?
a) NaCl and AgNO3
b) BaS and CuSO4
c) NH4OH and CuSO4
Reactions involving ions can be written in three ways:
1. Molecular (Formula) equation:
Dissolved salts are indicated with (aq).
Individual ions are NOT shown.
Na2CO3(aq)+ CaCl2(aq)CaCO3(s)+2NaCl(aq)
What does Na2CO3(aq) mean?
2. Complete Ionic Equation shows all dissolved species as separate ions.
It is written after the molecular equation and is NOT simplified.
Na2CO3(aq)+ CaCl2(aq)CaCO3(s)+2NaCl(aq)
Becomes:
Ions that are present in the solution but do not participate in the reaction are called spectator ions.
3. NET Ionic reaction. Only ions that change are included in the net ionic equation. Spectator
ions are left out
Example 3: Write the molecular equation, the complete ionic equation, and the net ionic
equation for any precipitate that forms if silver nitrate is mixed with calcium chloride.
pp. 83-86, # 21-25
III. Solubility — Selective Precipitation
Selective Precipitation can qualitatively determine the absence or present of certain ions.
Components in a mixture can be determined by their precipitates.
Combinations of ions that precipitate with only one component of a mixture must be found.
Example 1:
A solution could contain iron ions and/or calcium ions. How can you determine which ions are
present?
Example 2:
A solution may contain silver ions, Strontium ions or barium ions. How can you determine
which are present?
Hard water is water with Ca2+ and/or Mg2+ ions dissolved in it.
These ions dissolve through the action of acid rain (natural or man-made), on CaCO3 (limestone)
and MgCO3, causing them to dissolve slightly.
Hard water tastes bad, and can precipitate inside of pipes and equipment.
Hard water is usually softened by precipitating the ions and filtering the water. Washing Soda
Na2CO3 is often used to form CaCO3(s) and MgCO3(s).
Some water contains HCO3– ions. These will form CO32– ions if the water is boiled, and soften
the hard water without adding additional chemicals. This is called temporarily hard water.
p. 90, # 26-36(even), p. 104, # 76-80
IV. Solubility — Ksp
Quantitatively Solubility is an equilibrium with an equilibrium constant.
Solubility equilibrium are written with the solid as the reactant and ions as products.
AB(s)  A+(aq) + B–(aq)
Ksp = [A+][B–]
AB2(s)  A+(aq) + 2B–(aq)
Ksp = [A+][B–]2
“sp” stands for solubility product.
Solubility — amount required to make a saturated solution.
Molar Solubility — Molar concentration of a saturated solution
Solubility Product (Ksp) — is the value of the equilibrium expression that describes the
saturated solution.
These concepts are related, but they are not the same thing. BE CAREFUL.
The size of Ksp is an indicator of the solubility of a compound.
As Ksp decreases, the equilibrium favours __________ more, has fewer _______, and is less
soluble.
Example 1:
Which is the least soluble?
CaCO3
CaC2O4
CaSO4
Example 2:
A solution of Calcium ions, fluoride ions and Calcium fluoride solid is at equilibrium. [Ca2+] =
3.2 x 10-5M, [F–] = 7.2 x 10-7 M. What is the value of the equilibrium constant?
Q’s 40, 41, 42
V. Solubility — Ksp II
Solubility equilibrium use an “ICE” table to calculate how a system shifts to get to equilibrium.
This will most often be used if the compound is the source of the ions in the solution.
Example 1:
What is the solubility in g/L of silver chloride?
Example 2:
What is the molar solubility of copper(II)iodate?
Example 3:
How many grams of strontium fluoride will dissolve in 5.0 L of water?
Example 4:
2.50 g of lead(IV)sulphite will dissolve in 3.00 L of water. What is the equilibrium constant for
lead(IV)sulfite?
Example 5:
What is the relationship between Ksp and molar solubility (s) for a salt of the following type:
a) AB(s)
b) A2B(s)
c) AB2(s)
p. 95, # 43, 44, 46-49, 51-53
VI. Solubility — Ktrial
If two dilute solutions with an insoluble combination of ions are mixed, there is a chance a
precipitate will form.
To determine if a precipitate forms, we calculate a Ktrial. This will tell us if there are enough ions
to form a precipitate.
If Ktrial < Ksp then there are not enough ions to be at equilibrium —
If Ktrial = Ksp then there are just enough ions to be at equilibrium —
If Ktrial > Ksp then there are too many ions to be at equilibrium —
Note: If two solutions are mixed, all ions will be diluted due to the increase in volume of the
combined solution.
A dilution calculation will be the first step.
For Ksp, the Ktrial is called “Q” or TIP for trial ion product.
Example 1:
25 mL of a 3.5 x 10-6 M AgNO3 solution is mixed with 55 mL of a 6.5 x 10-5 M NaCl solution.
Does a precipitate form?
Example 2:
2.5 L of 0.0048 M Pb(NO3)2 is mixed with 3.5 L of 0.0031 M CaI2. Will a precipitate form?
Example 3:
What concentration of hydroxide ions is required to precipitate in a 2.5 x 10-9 M Fe3+ solution?
pp. 98-99, # 57-69(odd)
VII. Solubility — Common Ions Effect
We can alter the solubility of a salt be shifting the equilibrium by adding an ion common to the
equilibrium, or otherwise affecting an ion in the equilibrium.
Shift left —
Shift right —
Example 1: What is the effect of the following on the solubility of AgCl:
AgCl(s)  Ag+(aq) + Cl–(aq)
a)
Add AgNO3
b)
Add NaCl
c)
Add NaBr
d)
Add AgCl
e)
Add water (careful)
f)
Add NaNO3
g)
Add Pb(NO3)2
Example 2: What is the effect of the following on the solubility of Al(OH)3
Al(OH)3(s)  Al3+(aq) + 3 OH–(aq)
a)
Add AlCl3
b)
Add NaCl
c)
Add HCl
d)
Add NaOH
e)
Add water
f)
Add NaNO3
Example 3: Arrange the following solutions in order of increasing solubility of AgCl(s)
a)
pure water
b)
1 M NaCl
c)
1 M CaCl2
d)
2 M NaCl
e)
1 M AgNO3
f)
1 M FeCl3
g) 1 M NaBr
p.108 # 81-86
VIII. Solubility — Titrations
Chloride ions can be titrated with a silver nitrate solution to determine the moles of chloride ions
present.
Ag+ (aq) + Cl–(aq)  AgCl(s)
The indicator for this titration is chromate ions. Silver chromate forms a red precipitate.
Silver chromate is more soluble than silver chloride.
After most of the chloride ions have precipitated with the silver ions, then the silver chromate
will precipitate forming a red precipitate, indicating the chloride has mostly reacted.
Answer titration questions using the following steps:
1) Write the reaction.
2) Indicate the information given in the question.
3) Perform the stoichiometric calculation.
4) Check to make sure the question is answered.
Example 1:
A 50.00 mL sample of Langley water is titrated with 17.28 mL of a 0.0150 M Ag+ solution, with
a chromate indicator. What is the chloride concentration of the original water sample.
Example 2:
A 10.00 g silver coin is dissolved into 0.5000 L of nitric acid. 25.00 mL of this solution is
titrated with 17.45 mL of 0.010 M Cl– solution. What is the percent purity of the silver coin?
p. 101 # 70-74