Dr. Williamson’s KSP notes © vm williamson

Dr. Williamson’s KSP notes© vm williamson

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Ksp

Slightly Soluble Salts

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Dr. V. M. WilliamsonSTUDENT VERSION

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Solubility

Copyright © 1995 by Saunders College Publishing

Solubility: the _________ amount of solid that will dissolve in a given volume of solvent

Weight solubility

Molar solubility

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UnsaturatedSaturated

SupersaturatedSolutions


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Some Terms to Remember! “Soluble” in a particular solvent: at

least ____ mol/L of solid dissolves; solids with lower solubilities are slightly soluble or insoluble

! unsaturated solution: amount dissolved _____solubility

! saturated solution: amount dissolved _____ solubility

! supersaturated solution: amount dissolved _____solubility

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Solubility Equilibrium! When solid AgC2H3O2 is put in water, some

dissolves and dissociates into ions:AgC2H3O2(s) " Ag+(aq) + C2H3O2–(aq)

! As dissolution proceeds and ionic concentrations build up, the ions collide and reform the solid:

Ag+(aq) + C2H3O2–(aq) " AgC2H3O2(s) ! In time, the rate of the two competing

processes become equal; no more solid dissolves–solution is ___________; it is in a state of_______________________________

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Solubility Equilibrium! This dynamic equilibrium occurs in a

_________________ solution of AgC2H3O2(s) :AgC2H3O2(s) #" AgC2H3O2 (aq)

! AgC2H3O2(s) #" Ag+(aq) + C2H3O2–(aq)! The equilibrium constant for this system is

called the solubility product constant, Ksp:Ksp =

! and are equilibrium concentrations expressed in M (mol/L); these concentrations vary depending on the conditions

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Precipitation of AgClCopyright © 1995 by Saunders College Publishing

Ag+(aq) + Cl–(aq) #" AgCl(s)

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Solubility of PbCl2


PbCl2(s) #" Pb2+(aq) + 2Cl–(aq)

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Compare Methods! To Completion! ICF! Limiting Reagent! Moles! A + B if on strong,

not buffer! SA! SB! Soluble Salts

! In Equilibrium! ICE! Equilibrium Const! Molarity! Buffer from Weak

A or B! WA ! WB ! Slightly Soluble

Salts (Insoluble)

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Expression for the Solubility Product Constant of Slightly Soluble Salts

! BaSO4(s) #" Ba2+(aq) + SO4 2–(aq)Ksp =

! Bi2S3(s) #" 2Bi3+(aq) + 3S2–(aq)Ksp =

! MgNH4SO4(s) #" Mg2+(aq) + NH4+ (aq) + SO42–(aq)

Ksp =

= 1.1 x 10–10

= 1.6 x 10 –72

= 2.5 x 10–12Numerical values from a Ksp Table

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Selected Ksp Values

Substance KspAluminum compoundsAl(OH)3 1.9 x 10–33

AlPO4 1.3 x 10–20

Barium compoundsBa3(AsO4)2 1.1 x 10–13

BaF2 1.7 x 10–6

Iron compoundsFeCO3 3.5 x 10–11

Fe4[Fe(CN)6]3 3.0 x 10–41

Usually at 25˚C and in 2 sig figs

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Solubility and Ksp

WeightSolubility of Compound (g/L)

Molar Solubility of Compound (mol/L)

Molar Concentrationof Ions (mol/L)Ksp

Molar Mass

Formula

Ksp Expression

.s__________s _________ ____________

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Calculate Ksp from Solubility Data! 0.746 g of AgC2H3O2 dissolves in 100 mL of

solution at 25˚C. Calculate the Ksp.! Note that you are given information to find

the ‘x’ amount! AgC2H3O2 (s) #" Ag+ (aq) + C2H3O2– (aq) ICE

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Calculate Ksp from Solubility Data! 0.746 g of AgC2H3O2 dissolves in 100 mL of

solution at 25˚C. Calculate the Ksp.

[Ag+] = ___________M and [C2H3O2–] = _______ M

Ksp = [Ag+] [C2H3O2–] =

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Using the Solubility Product! When the Ksp is known, the solubility of the

compound in water at 25oC can be determined. For example:

! The Ksp for CaF2 is 3.9 x 10 –11 . What is the molar solubility of CaF2?

! Plan:

Formula Ksp Expression

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Calculate the Molar Solubility! The Ksp for CaF2 is 3.9 x 10 –11 . What is the

molar solubility of CaF2?! CaF2(s) #" Ca2+(aq) + 2F–(aq)

ICE

Ksp =Ksp =

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Calculate the Solubility by Mass! The Ksp for CaF2 is 3.9 x 10 –11 . What is the

solubility of CaF2in water in grams per liter?Molar Solubililty of CaF2 =

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Ksp and Molar Solubility

BaSO4AgCl

Ag2CrO4

Substance

1.1 x 10 –10

1.8 x 10 –10

9.0 x 10 –12

Ksp*

*For aqueous solutions at 25oC

For compounds with same stoichiometry, the one with larger Ksp has higher solubility

Solubility(mol/L)

1.1 x 10 –5

1.34 x 10 –5

1.3 x 10 –4

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Relationships

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Using the Solubility Product! A salt A2B has a solubility of 3.2 e-3 g/L of

solution and a molar mass of 42 g/mol. Find the Ksp.

3.2 e-3 g/L ( ) =A2B(s) #" 2A+ (aq) + B2 – (aq)

Ksp = [A+]2 [B2-] = ( ) ( )

Ksp =

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The molar solubility of CaF2 in pure water at 25oC is 2.1 x 10–4 mol/L.

How would its solubility change if it were dissolved in a 0.010 M NaF solution?

NaF(s) " Na+(aq) + F–(aq)0.01 M 0.01 M 0.01 M

How would this affect the equilibrium??CaF2(s) #" Ca2+(aq) + 2F–(aq)

Has _____________, so equilibrium should shift ______________________________________

Common Ion Effect: Calculations

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! CaF2(s) #" Ca2+(aq) + 2F–(aq)ICE

! Let x = molar solubility of CaF2

! Substitute x into Ksp expression and solve for x:! Ksp =! Approx:! x = ______________M = the molar solubility of

CaF2 in 0.010 M NaF

Common Ion Effect: Calculations

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________ M = the molar solubility of CaF2 in 0.010 M NaF, which is ___than the molar solubility in water ( ____________M)

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! Calcium in blood as:CaHPO4(s) #" Ca2+(aq) + HPO4

2–(aq)! Pop has H3PO4, a weak acid! H3PO4(aq) #" H+(aq) + H2PO4

–(aq)! H2PO4

– (aq) #" H+(aq) + HPO42–(aq)

! Will heavy pop drinkers have:(A) (B) (C)

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You Predict:! Consider the effect of each on the

solubility of Mg(OH)2, a slightly soluble base.

! Adding _________! Adding _________! Adding _________! Adding _________Vote on each:

(A) Increased solubility(B) Decreased solubility(C) No change in the

solubility

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Common-Ion Effect: Demonstration

PbCl2(s) #" Pb2+(aq) + 2Cl–(aq)NaCl(s) " Na+(aq) + Cl–(aq)


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The Reaction Quotient! For the dissolution/precipitation equilibrium

described by:MyXz(s) #" yMz+(aq) + zXy–(aq)

Ksp = [M z+]y [Xy–]z

_____________ concentrations

Qsp = [M z+]ney [Xy–]ne

z

______________concentrations

Qsp Ksp : unsaturated; solid dissolvesQsp Ksp : saturated; equilibriumQsp Ksp : supersaturated; solid forms

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Using the Reaction Quotient! The reaction quotient can be used to predict

whether precipitation is going to occur:

+ ?

100. mL0.0100MAgNO3

50.0 mL0.0100 MKIO3

Calculate the concentrations of all the ions and evaluate Qsp

AgIO3solid

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Using the Reaction Quotient! Using M1V1 = M2V2, the new concentrations are:

[AgNO3] =

! [KIO3] =

! Equilibrium Equation is:AgIO3(s) #" Ag+ (aq) + IO3– (aq)

! Ksp = [Ag+ ][IO3– ]

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Using the Reaction Quotient! Compare Qsp to Ksp :

Qsp = [Ag+ ][IO3– ]

= =

From Table: Ksp = 3.0 x 10–8

! Since Qsp Ksp , AgIO3 should

! Rule of thumb: a precipitate will be visible to the naked eye when the ratio Qsp/Ksp ≥ 1000

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Predicting Precipitation:A Summary

! Determine if a reaction that forms a slightly soluble or insoluble solid will occur

! If a slightly soluble or insoluble solid is formed, determine initial concentrations of all ions in solution

! Calculate Qsp and compare with Ksp: $ the solid should precipitate out of solution if Qsp Ksp, $ more will dissolve if Qsp Ksp, or $ the system is saturated and at equilibrium if Qsp Ksp

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Changing pH

Co(OH)2(s) #"Co2+ (aq) + 2OH– (aq)

Explain why adding acid changed the solubility.

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pH! To a solution of pH = 6.00, a sufficent

amount of Ce(NO3)3 is added to make the total [Ce3+] = 0.00300 M. Will Ce(OH)3precipitate?? Ksp Ce(OH)3 = 1.5 e-20

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CaF2(s) #"Ca2+ (aq) + 2F– (aq)

Molar solubility in water = 2.1 e-4pH = 7.00

Why does solubility go UP with addition of an Acid or Decreasing pH??

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Fractional Precipitation! Allows the selective removal of ions

from solution based on differences in the solubilities of their saltsCompound Solubility ProductAgCl 1.8 x 10–10

AgBr 3.3 x 10–13AgI 1.5 x 10–16

● _______expected to precipitate first followed by AgBr, and then _______

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SSS Equilibrium Shifts! A common ion is added _________ ! If no, does a reaction occur between

ions added and ions in the SSS ?$If WA or WB are formed ________$If water is formed ________$If another SSS is formed ________$Is a SA, SB, Soluble Salt formed

________

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Dr. Williamson’s KSP notes © vm williamson