Goes with chapter 19: Silberberg’s Principles of General Chemistry

AP ChemistryMrs. Laura Peck, 2013

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Objectives/Study GuideWrite balanced equations for the dissolution of a salt and its

corresponding solubility product expression.Predict the relative solubilities of salts which dissolve to give

the same number of ions from their Ksp valuesCalculate the Ksp value from the solubility of a salt and also

calculate the solubility of the salt in units of mol/L or g/L from the given Ksp value

Predict the effect of a common ion on the solubility of a salt and perform calculations.

Perform calculations to predict if a precipitate will form when two solutions are mixed

Do problems involving selective precipitation.Perform calculations involving complex ions and solubilityUse qualitative analysis to separate a mixture of ions

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AP tip:Solubility problems would appear in the first

free-response question, since they would be considered an equilibrium problem. Awareness of the types of problems outlined in this Topic and the methods used to solve them will lead you to succeed in this topic on the AP exam.

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Solubility ProductYou should have already memorized the solubility rules and know

which salts are soluble. For slightly soluble or insoluble salts, an equilibrium exists between

the solid and its aqueous ions. Ex: PbCl2(s) Pb2+

(aq) + 2Cl-(aq)

At first, when the salt is added to the water, there are no ions present. As the solid dissolves, the concentration of the ions increases. A simultaneous competing process is the reverse of the dissolution, that is, the

reforming of the solid called crystallization. At some point, the maximum amount of dissolution is achieved, which is called

the saturation point. However, remember that on a molecular level, a dynamic equilibrium exists

between dissolved solute and undissolved solid (Rdissolution=Rcrystallization) The solution is saturated when no more solid dissolves and equilibrium is

reached. Keq = Ksp = [Pb2+][Cl-]2

The constant, Ksp, is the solubility product constant. For salts producing the same number of ions, the Ksp value can be

used to measure the extent to which the solid dissolves. The larger the Ksp value, the more soluble the salt.

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Example #1Given the following salts and their Ksp values,

which salt is the most soluble? Which salt is the least soluble?

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Formula Ksp

NiCO3 1.4x10-7

MnS 2.3x10-13

CaSO4 6.1x10-5

The most soluble salt is the salt with the largest Ksp value, CaSO4

The least soluble salt is the salt with the lowest value of Ksp, MnS. You are able to compare the Ksp values to determine the relative solubilities of the salts because they all produce the same number of ions.

Calculations involving solubilityThe solubility of a salt that will dissolve in 1 L of

water. The solubility of a salt can be given in units of mol/L

or g/L. The solubility of a salt can be used to determine the

Ksp value for the salt.

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Example #2: Calculating Ksp from solubility. The solubility of Pb3(PO4)2 is 6.2x10-12M.

Calculate the Ksp value for the solid.

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Step 1: write the reaction for the dissolution of the solid.

Pb3(PO4)2(s) 3Pb2+(aq) + 2PO43-

(aq)Step 2: Underneath the reaction, make an ICE chart.

I. 0 0C. +3x +3xE. 3x 3x

In the ICE chart, x represents x mol/LOf Pb3(PO4)2(s) dissolving to reachKeq which equals 6.2x10-12 M

For every 1mol/L of Pb3(PO4)2, whichDissolves, 3 mols/L of Pb2+ and 2 mol/LOf PO4

3- form

3x is the mol/L of Pb2+ produced when The solid Pb3(PO4)2 dissolves

2x is the mol/L of PO43- produced when

The solid, Pb3(PO4)2 dissolves

Step 3: Write the Keq expression for the reactionAnd plug in the values from the E line of the ICE

Keq = Ksp = [Pb2+]3[PO43-]2 = (3x)3(2x)2 = 108x5

The value of x is the solubility of Pb3(PO4)2, whichEquals 6.2x10-12M

Ksp = 108(6.2x10-12)5 = 9.9x10-55

Or you can use stoich to solve this problem:

6.2x10-12 x molPb3(PO4)2 x 3mol Pb2+ = 1.9x10-11M Pb2+

1 L 1mol Pb3(PO4)2

6.2x10-12 x mol Pb3(PO4)2 x 2 mol Pb43- = 1.2x10-11 M PO4

3-

1 L 1mol Pb3(PO4)2

Plug these values into the Ksp expression and solve for Ksp:

Ksp = [Pb2+]3[PO43-]2 = (1.9x10-11)3(1.2x10-11)2 = 9.9x10-55

Calculating solubility from KspIf you are given a group of salts which do not

all have the same cation to anion ratio and asked which is more soluble, you must perform a calculation to determine the solubility of each salt.

If the salts each provide a number of ions in solution, so you cannot directly compare the Ksp values to predict which is more soluble.

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Example #3: Given the two salts in the table below, which is more soluble? Show calculation to support your answer. Salt Ksp

FeC2O4 2.1x10-7

Cu(IO4)2 1.4x10-7

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Step 1: Write the reaction for the dissolution of the solid Salt #1

Step 2: Underneath the reaction, make an ICE chart

Step 3: Write the equilibrium expression for Ksp, plug in The equilibrium line, and solve for x

Step 4: Repeat process for salt #2

FeC2O4(s) Fe2+(aq) + C2O4

2-(aq)

I. 0 0C. +x +xE. X x

Ksp = [Fe2+][C2O42-] 2.1x10-7 = x2 x = 4.6x10-4 mol Fe2+/L

4.6x10-4 mol Fe2+ x 1mol FeC2O4/1L = 4.6x10-4 mol FeC2O4/L

Cu(IO4)2(s) Cu2+(aq) + IO4

-(aq)

I. 0 0C. +x +2xE. X 2x

Ksp = [Cu2+][IO4-]2 1.4x10-7 = x(2x)2 4x3 = 1.4x10-7 x = 3.3x10-3 mol/L

3.3x10-3mol Cu2+ x 1mol Cu(IO4)2/1L = 3.3x10-3mol Cu(IO4)2/LTurns out that Cu(IO4)2/L is greater than FeC2O4/L so its more soluble!

Common Ion EffectWhen a salt is dissolved in water containing a

common ion, its solubility is decreased. Consider the solubility equilibrium of silver

sulfate:Ag2SO4(s) 2Ag+

(aq) + SO42-

(aq)

When silver sulfate is dissolved in 0.100M AgNO3, the Ag+ ion from silver nitrate causes the equilibrium to shift to the left, decreasing the solubility of silver sulfate.

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Example #4Calculate the molar solubility of Ag2SO4 in

0.10M AgNO3. Ksp for Ag2SO4(s) is 1.2x10-5

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First, fill out the ICE chart. Be sureTo include the initial [ ] of the ionsFrom the soluble salt, AgNO3.

Second, plug in the equilibrium lineInto the Ksp expression.

Ag2SO4(s) 2Ag+(aq) + SO4

2-(aq)

I… 0.10 0C… +2x +xE… 0.10+2x x

Ksp = [Ag+]2[SO42-] = (0.10 + 2x)2(x)

Assume that 0.10 x 2x is about 0.10, since Ksp is small you can assume that the change (2x) from the initial concentration (0.10) is negligible.

Ksp = 1.2x10-5 = (0.10)2(x) x = 1.2x10-3 mol SO42-/L

1.2x10-3mol/L SO42- x (1mol Ag2SO4)/(1mol SO4

2-) = 1.2x10-3mol/L Ag2SO4

The solubility of silver sulfate in 0.100M silver nitrate, 1.2x 10-3 mol/L is lessThan the solubility of silver sulfate in pure water, 1.4x10-2 mol/L

pH and SolubilityChromium (III) hydroxide dissolves according to the

equilibrium:Cr(OH)3(s) Cr3+

(aq) + 3OH-(aq)

An increase in pH, caused by the addition of OH- ions, will shift the equilibrium to the left, decreasing the solubility of Cr(OH)3

A decrease in pH, caused by the addition of H+ ions, will shift the equilibrium to the right, increasing the solubility of Cr(OH)3.

The H+ ions remove the OH- ions from the solution. A salt with the general formula, MX, will show increased

solubility in acidic solution if the anion, X-, is an effective base (if HX is a weak acid).

Common anions that make effective bases include S2-, OH-, and CO32-

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Example #5Calculate the solubility of

Fe(OH)3 in a solution with a pH equal to 5.0. Ksp = 4.0x10-

38

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First steps are to write out the reactionThen fill out an ICE chart. CalculateThe initial hydroxide concentrationFrom the pH.

Second, plug E values into Ksp equation

pOH = 14 – pH = 14.0-5.0 = 9.0

[OH-] = 1.0x10-9 M

Fe(OH)3(s) Fe3+(aq) + 3OH-

(aq)

I.. 0 1.0x10-9

C.. X 3x E.. X 1.0x10-9

Ksp = 4.0x10-38 = [Fe3+][OH-]3 = x(1.0x10-9)3

Because pH is 5.0, the equilibrium[OH-] must be equal to 1.0x10-9

X = 4.0x10-11 M

Precipitate formation

A precipitate may or may not form when two solutions are mixed, depending on the concentration of the ions involved in the formation of the solid.

Ion Product: The ion product, Q, is written in the same way as the Ksp

expression. F0r Lead(II) chloride, Q = [Pb2+][Cl-]2

Calculation of the value, Q, involves the use of the initial concentrations of the solutions mixed, [Pb2+]0 and [Cl-]0, instead of the equilibrium concentrations.

A comparison of the value of Q to Ksp determines if a precipitate is formed. Q>Ksp – precipitation occurs Q<Ksp – no precipitation occurs Q=Ksp – the solution is saturated.

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Example #6Will a precipitate for when 100.0

mL of 4.0x10-4M Mg(NO3)2 is added to 100.0 mL of 2.0x10-4M NaOH?

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Step 1: Determine the identity of the precipitate formed.

Step 2: Determine the concentration of the ions after they are Mixed and before any reaction occurs.

Determine the moles of concentration of each solute present. Be sure to divide by the total volume of the two solutions mixed.

Step 3: Write the ion product expression, calculate its value, andCompare it to Ksp, which equals 8.8x10-12

Mg(OH)2 is the possible precipitate. NaNO3 is always soluble.

[Mg2+]0 = (0.100L x (4.0x10-4mol/L))/0.200L = 2.0x10-4M

[OH-]0 = (0.100L x(2.0x10-4mol/L))/0.200L = 1.0x10-4M

Q = [Mg2+][OH-]2 = (2.0x10-4M)(1.0x10-4)2

= 2.0x10-12

Since Q<Ksp – no precipitate will form

Selective PrecipitationA reagent is added to a mixture of metal ions, thus

forming a precipitate. One metal ion will precipitate before the other, allowing

the mixture to be separated. Example #7: a solution contains 0.25M Ni(NO3)2 and

0.25M Cu(NO3)2. A solution of Na2CO3 is slowly added to this solution.

A) Will NiCO3 (Ksp = 1.4x10-7) or CuCO3 (Ksp = 2.5x10-10) precipitate first?

B) Calculate the concentration of CO32- necessary to begin

the precipitation of each salt.

C) Determine the concentration of Cu2+ when NiCO3 begins to precipitate. 16

CuCO3 will precipitate first because its Ksp is smaller

For CuCO3 precip. begins: [CO32-]= Ksp/[Cu2+] = 2.5x10-10/0.25M = 1.0x10-9M

For NiCO3 precip. Begins: [CO32-]=Ksp/[Ni2+]=1.4x10-7/0.25M = 5.6x10-7M

[Cu2+] = Ksp/[CO32-] = 2.5x10-10/5.6x10-7M = 4.5x10-4M

Complex Ion Equilibria

A complex ion consists of a metal ion surrounded by ligands which are Lewis bases such as H2O, OH-, NH3, Cl-, and CN-

The coordination number is the number of ligands which attach to the transition metal ion.

Common coordination numbers include 4 for Cu2+ and Co2+

and 2 for Ag+

The ligands attach one at a time to the metal ion; each step has a formation constant, Kf: Ag+ + S2O3

2- Ag(S2O3)- Kf1 = 7.4x108

Ag(S2O3)- + S2O32- Ag(S2O3)2

3- Kf2 = 3.9x104

Formation of a complex ion causes a precipitate to dissolve. The equilibrium, AgBr(s) Ag+ + Br- is not affected by

the addition of H+. But the concentration of Ag+ can be lowered by the addition

of excess S2O32- forming the complex ion, Ag(S2O3)2

3-

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Example #8 Calculate the mass of AgBr that can dissolve in 1.00L of 0.500M Na2S2O3. Ksp for AgBr = 5.0x10-13

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Step 1: Determine the overall reaction by Adding up the reactions for the solubilityEquilibrium and the stepwise formation Of the complex ion. Remember, to get the Equilibrium constant for the overall reaction,K, multiply together the K for each step. (from Preceding slide)

Step 2: Complete an ICE chart with the overall reaction

Step 3: Plug E line into Ksp expression

AgBr(s) Ag+(aq) + Br-(aq) Ksp =

5.0x10-13

Ag+ + S2O32- Ag(S2O3)-

Kf1 = 7.4x108

Ag(S2O3)- + S2O32- Ag(S2O3)2

3- Kf2 = 3.9x104

AgBr(s) + 2S2O32- Ag(S2O3)2

3- + Br- K’=14.4

I.. 0.500 0 0C.. -2x +x +xE.. 0.500-2x x x

K’ = 14.4 = [Ag(S2O3)23-][Br-]/[S2O3

2-]2

= x2/(0.500-2x)2 take square root of both sides3.79 = x/(0.500-2x)X = 1.90-7.58x x = 0.221MBr = 0.221M AgBr(s)

1.00L x (0.221mol AgBr/1L) x (187.8g/1mol) = 41.5g AgBr = 42g AgBr

Qualitative AnalysisQualitative analysis involves separating a mixture

of cations or anions based on their solubilities. Cations can be separated into five major groups

based on their solubilities. Group I: insoluble chloridesGroup II: sulfides soluble in acidic solutionGroup II: sulfieds insoluble in basic solutionGroup IV: insoluble carbonatesGroup V: alkali metal and ammonium ions

Each of these groups can be treated further to separate and identify the individual ions.

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Example #9

Separate a mixture containing the Group I cations: Ag+, Hg2

2+ and Pb2+

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Add cold HCl(aq)

Heat

Add CrO42-

Add NH3(aq)

Add H+

The End

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