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SOLUBILITY PRODUCT CALCULATIONS
At the end of this unit the student will be able to :
1- Write the Ksp expression for the ionization of any salts.
2- Calculate Ksp from solubility and vice versa.
3- Tell if a precipitate will form when mixing solutions. 4- predict whether it is possible to separate metal ions by the precipitation of its hydroxides or salts.
5- predict whether a complexing agent will dissolve the metal precipitate or not. 6- Understand the concept of common ion effect.
Objectives
Last update : 1/5/2014 SOLUBILITY PRODUCT CALCULATIONS Subjects
The concept of solubility product is very useful in explaining many phenomenons. Various
fields in which it can be used are:-
- Calculation of solubility: If we know the solubility product of a meagerly soluble
salt like AgCl we can calculate the solubility of the salt and vice versa.
- In predicting the precipitation in reactions: If we know the solubility product of a
salt, we can find whether on mixing the solution of its ions, precipitation will occur or not.
- In inorganic qualitative analysis:
The concept of solubility product and common ion effect play a vital role in
the separation of basic radicals i.e. cations into different groups of qualitative analysis.
- Purification of sodium chloride: Sodium Chloride obtained from sea water or lakes is always impure. It can be purified on the basis of common ion effect.
Introduction
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The Solubility Product Concept
The state of overrsaturation is temporary ( less than one second ) because Cl-^ and Ag+^ will react to form the precipitate AgCl and the [Cl-] and [Ag+] will go back to the saturation state .So when you see a solution of AgCl with a solid in it you will know that the product of [Cl-]X[Ag+] dissolving in solution is equal to the Ksp of AgCl. What we said about AgCl applies to all salts. To write the expression of Ksp for any salt you should first write the dissociation equation , for example the dissociation equation for the salt Al 2 S 3 is :
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Last update : 1/5/2014 SOLUBILITY PRODUCT CALCULATIONS Subjects
Al 2 S 3 → 2 Al3+^ + 3 S2- The Ksp expression for Al 2 S 3 is the product of the concentrations of the ions, with each concentration raised to a power equal to the coefficient of that ion in it's balanced dissociation equation thus : Ksp = [Al3+^ ]^2 [S2-^ ]^3
and do the same with any other salt e.g for CaF 2 , Ksp = [Ca2+^ ][F-^ ]^2 and so on.
Saturated solution of CaF 2
The Solubility Product Concept
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Effect of temperature on solubility : Generally in most cases solubility increases with the rise in temperature. However we must follow two behaviors : In endothermic process solubility increases with the increase in temperature and vice versa. In exothermic process solubility decrease with the increase in temperature. Gases are more soluble in cold solvent than in hot solvent
Factors Affecting Solubility ( Temperature )
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Effect of solvent on solubility :
Solubility of a solute in a solvent purely depends on the nature of both solute and solvent. A polar solute dissolved in polar solvent. A polar solute has low solubility or insoluble in a non-polar solvent. For this reason if you want to decrease the solubility of an inorganic salt ( polar salt ) in water you mix the water with an organic solvent ( non polar ).
Factors Affecting Solubility ( Solvent )
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Q is greater than Ksp so a precipitate of lead(II) chromate will form. If Q ≤ Ksp the solution will be either saturated ( if equal) or unsaturated ( if less than ) and in both states there will be no precipitation. Q < Ksp Unsaturated solution Q = Ksp Saturate solution Q > Ksp Oversaturate solution
Predicting Precipitation
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What is the solubility of a salt? It is the maximum amount ( moles , mmoles , g …etc ) of this salt that can be dissolved in a certain volume of solution ( L , 50 mL , 500 mL …ect.). The molar solubility is the solubility in moles of a salt in liter of a solution.
Calculating Ksp of a salt from it’s Solubility : From the definition of Ksp and molar solubility one can calculate the Ksp of a salt from it’s molar solubility or vice versa.
Calculating Ksp From Solubility
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PbCl 2 (s) → Pb2+(aq)^ 2Cl-(aq)
Initial Concentration ( I )
0.0159 moles/L 0 0
Complete dissolution ( C )
0 + 0.0159 M + 0.0318 M
Fourth, substitute these concentrations into the equilibrium expression and solve for Ksp. :
Calculating Ksp From Solubility
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Ksp = [Pb2+^ ][Cl-^ ]^2 = [0.0159][0.0318]^2 = 1.61 x 10-
Calculating the Solubility of a salt from its Ksp :
Example : Estimate the molar solubility of Ag 2 CrO 4 in pure water if the solubility product constant for silver chromate is 1.1 x 10-12^?
Solution : Ag 2 CrO 4 (s) --> 2 Ag+(aq) + CrO 4 2-(aq) Ksp = [Ag+]^2 [CrO 4 2-]
Let "x" be the number of moles of silver chromate that dissolves in one liter of solution (its molar solubility).
Calculating Solubility from Ksp
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Effect of the common ion on solubility : The solubility of an ionic compound decreases in the presence of a common ion. A common ion is any ion in the solution that is common to the ionic compound being dissolved. For example, the chloride ion in a sodium chloride solution is common to the chloride in silver chloride. The presence of a common ion must be taken into account when determining the solubility of an ionic compound. To do this, simply use the concentration of the common ion as the initial concentration.
Effect of the Common Ion on Solubility
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Let "x" represent the barium sulfate that dissolves in the sodium sulfate solution expressed in moles per liter Make an "ICE" chart..
Example: Estimate the molar solubility of barium sulfate in a 0.02 M sodium sulfate solution. The solubility product constant for barium sulfate is 1.1 x 10-10^?
Solution : BaSO 4 (s) --> Ba2+(aq) + SO 4 2-(aq) Ksp = [Ba2+][SO 4 2-]
Effect of the Common Ion on Solubility
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Salt effect (ionic strength): Having an opposing effect on the Ksp value compared to the common ion effect, uncommon ions increase the Ksp value. Uncommon ions are those that are different from those involved in Ksp equilibrium. The figures on your right show the effect of KNO 3 on the solubility of BaSO 4. As you see K+ ions surround SO 4 2-^ and NO 3 -^ ions surround Ba2+^ ions. Therefore , Ba2+ ions will have difficulty reacting with SO 4 2-^ to form the precipitate BaSO 4.
Effect Of Ionic Strength On Solubility
Last update : 15/2014 SOLUBILITY PRODUCT CALCULATIONS Subjects
Effect of pH on Solubility : Many weakly soluble ionic compounds have solubility which depend on the pH of the solution e.g metal hydroxides and the salts of weak acids. 1- Effect of pH on metal hydroxides :
Example : Zinc hydroxide Zn(OH) 2 has Ksp= 4.5× 10 -
In pure water calculate its malor solubility? Solution : Assume the molar solubility = x
Zn(OH)2(s) Zn2+(aq) +2 OH-(aq) I x 0 0 C 0 x 2x
Effect of pH on Solubility
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