SolubilityLesson 6

Changing solubility/Common Ion Effect

Some quick review on Ksp and TIP calculations

Remember to check where your source is for the ion in the precipitate eqn.  1. The molar solubility of Ba(OH)2 is 2. Equal volumes of 0.0010 M Ba(NO3)2 and

0.0010 M. Ksp = ? 0.0010 M NaOH are mixed. Will a

precipitate occur?

Multiply by 2 because of Ba(OH)2(s) Do not multiply by 2 cuz it comes from NaOH

TIP < Ksp No ppt.Ksp = 4.0 x 10-9

Ksp = 4[0.0010]3TIP = 1.25 x 10-10

Ksp = 4s3

Ksp = [s][2s]2

Ksp = [Ba2+][OH-]2 TIP = [Ba2+][OH-]2

TIP = [0.00050][0.00050]2

0.00050 M 0.00050 M

12

0.0010 M 0.0010 M12

Ba(OH)2(s) ⇌ Ba2+ + 2OH-Ba(OH)2(s) ⇌ Ba2+ + 2OH-

s s 2s

3. AgNO3(s) is added to 1.0 L of a solution containing Cl- in order to

precipitate out the Cl- as AgCl(s). After the AgCl(s) has formed, what is the

concentration of Cl-?

AgCl(s) ⇌ Ag+ + Cl-

s s s

Ksp = s2

1.8 x 10-10 = s2

s = [Cl-] = 1.3 x 10-5 M

Can you change solubility?• Up to now, we have only considered pure water

as the solvent for our solutions. – as long as your solution uses water as a solvent,

there is no change in the solubility of the salt.

• However, using a solvent other than water, definitely has an effect on the solubility of the solute (solid).

Factors that Increase or Decrease Solubility

• The solubility of a substance depends on the presence of other ions (caused by a solvent other

than water) AND the temperature.

• However, only temperature can change Ksp value.

energy + PbCl2(s) ⇌ Pb2+ + 2Cl-

The molar solubility is the amount of PbCl2(s) in moles per litre that dissolves into ions to form a saturated solution.

increases the molar solubility

decreases the molar solubility

Only changing the temperature changes the value of Ksp. Adding some “substances” will however shift the equilibrium just like we learnt in Unit 2 until

the system re-gains balance again.

Solid equilibriums are usually endothermic.

For your info.

If you go to the right (where ions are), you are increasing solubility.

If you go to the left (where the solid is), you are decreasing solubility.

DECREASING the Solubility of a Salt

AgNO3

Substance Added Molar Solubility Ksp Effect

energy + AgCl(s) ⇄ Ag+ + Cl-

NaCl

Decrease

Decrease none

none

What we just added were soluble salts that dissolve into their respective ions. Notice that these soluble salts had one common ion with the

equilibrium (Ag, Cl). This is called the COMMON ION EFFECT.

INCREASING the Solubility of a Salt

Substance Added Molar Solubility Ksp Effect

energy + AgCl(s) ⇄ Ag+ + Cl-

Na2S

Pb(NO3)2

Increase

Increase

none

none

2Ag+ + S2- → Ag2S(s)

Pb2+ + 2Cl- → PbCl2(s)

Forming a precipitate will always drive the equilibrium to make more ions, therefore increasing solubility.

Changing the Solubility of a Salt

Substance Added Molar Solubility Ksp Effect

energy + AgCl(s) ⇄ Ag+ + Cl-

H2O

AgCl(s)

Increase Temperature

Decrease Temperature

none

none

Increase

Decrease

Increase

Decrease

none

none

moles/L does not change

solid

Changing the Solubility of a Salt Energy + Ca(OH)2(s) ⇄ Ca2+ + 2OH-

Ca(NO3)2 Decrease

IncreaseNa2SO4

NaOH

NaCl

Ca(OH)2(s)

Increase Temperature

Decrease Temperature

Decrease

none

none

Increase

Decrease

Increase

Decrease

none

none

none

none

none

Substance Added Effect on Molar Solubility Effect on Ksp

1. Mg(OH)2 will have the greatest solubility in:

Mg(OH)2(s) ⇌ Mg2+ + 2OH-

A. NaOH

B. Mg(NO3)2

C. H2O

OH- lowers solubility

Mg2+ lowers solubility

No effect on solubility

Ag+ increases solubility by forming a ppt with OH- driving the equilibrium to make more ions.

D. AgNO3

Summary

Increase solubility– Add solvent that would precipitate your existing

ions.

Decrease solubility– Add solvent with a common ion with your

existing ions.

Homework

Hebden: Pg 108 #81-86

We did not cover sections 3.8 and 3.9- Not responsible for them.