ANALYTICAL CHEMISTRY(Solubility and solubility product)

Dr.S.SURESH

Assistant Professor

Email:avitsureshindia@gmail.com

Solubility

• The solubility of a substance is the amount of that substance that will dissolve in a given amount of solvent. “Solubility” may be considered to be an equilibrium; the equilibrium is between solid and ions in solution. Any ionic solid is 100% ionized in aqueous solution; once it actually dissolves.

NaCl(s)

Na+(aq) + Cl-(aq)

Factors that affect solubility

• Temperature– Solubility generally increases with temperature.

• Common ion effect– Common ions reduce solubility

• pH of solution– pH affects the solubility of ionic compounds.

• Formation of complex ion– The formation of complex ion increases solubility

Solubility product

In general, solubility product, Ksp, is the mathematical product of its dissolved ion concentrations raised to the power of their stoichiometric coefficients. This statement is called the solubility product principle

MyXz (s) yMZ+ (aq) + zXY-(aq)

[ ] [ ] zyyz

sp XMK −+=

Solubility product constant Molar solubility of the ions

Solubility product

Solubility product constant (Ksp)

Most salts dissociate into ions when they dissolve. For example:

BaSO4(s) ⇌ Ba2+(aq) + SO42-(aq)

This equilibrium system may be described by the mass-action expression

Ksp = [Ba2+][SO42-]

BaSO4(s) ⇌ Ba2+(aq)+ SO4

2-(aq)

• The Ba2+ concentration and the SO42– concentration

are equal since for each BaSO4 unit that dissolves one Ba2+ and one SO4

2– ion form.

Solubility = s = [Ba2+] = [SO42–]

From the above equation we know that

Ksp = [Ba2+] [SO42–]

Ksp = s × s = s2

Ksp = [Ba2+] [SO42-]

solubility product

constant

.

Problems on solubility product

• Calculate the Ksp value of BaSO4 which has a solubility of 3.9×10 5‒ mol/L at 25°C.

BaSO4(s) ⇌ Ba2+(aq)+ SO4

2-(aq)

Ksp = [Ba2+] [SO42–]

Ksp = s × s = s2

= 3.9×10 5 ‒ × 3.9×10 5 ‒

= 1.52×10 9‒

Solubility and Solubility Products

(1) What is the solubility of AgCl if the Ksp is 1.6 x 10-10

AgCl(s) ⇌ Ag+(aq) + Cl-(aq)

Ksp = [Ag+][Cl-]

If s is the solubility of AgCl, then:

[Ag+] = s and [Cl-] = s Ksp = (s)(s) = s2 = 1.6 x 10-10

s =

s = 1.3 x 10-5 mol/L

-1010 x 1.6

• What is the solubility of CuS if the Ksp is 1.27× 10–45.

CuS ⇌ Cu2+ + S2–

Ksp = [Cu2+] [S2–]

= [s] [s]

= s2

s =

=

= 3.56 × 10–23M

spK

-4510 x 1.27

(1) What is the solubility of Ca(OH)2 if the Ksp is 7.9 x 10-6

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

1×s 2×s

[ ] [ ] 212sp OHCaK −+=

[ ] [ ] 21sp 21K ss ××=

)2()2()1(K sp sss ××=

)()221(K sp sss ×××××=3

sp 4K s=

34sK sp =6109.7, −×=spKBut

36 4109.7 s=× −

36

4109.7

s=× −

361097.1 s=× −

Ms 23 6 1025.11097.1 −− ×=×=

(1) What is the solubility of Ca(OH)2 if the Ksp is 7.9 x 10-6

Solubility and Solubility Products

What is the solubility of Mg(OH)2 if the Ksp is 1.6 x 10-10

Mg(OH)2(s) ⇌ Mg2+(aq) + 2 OH-(aq)

Ksp = [Mg2+][OH-]2 = 8.9 x 10-12

If the solubility of Mg(OH)2 is s mol/L, then:

[Mg2+] = s mol/L and [OH-] = 2s mol/L,

Ksp = (s)(2s)2 = 4s3 = 8.9 x 10-12

s = 1.3 x 10-4 mol/L

Different types of solution

Unsaturated solution: More solute can be dissolved in it.

Saturated solution: No more solute can be dissolved in it. Any more of solute you add will not dissolve. It will precipitate out.

Super saturated solution: Has more solute than can be dissolved in it. The solute precipitates out.