Exp. 29 Molar Solubility, Common Ion Effect p. 331

1
Exp. 29 Molar Solubility, Common
Ion Effect
p. 331

• To determine the molar solubility and the
  solubility constant of calcium hydroxide,
  Ca(OH)2.
• To study the effect of a common ion on the molar
  solubility of Ca(OH)2.

2
Exp. 29 Introduction

• Dynamic Equilibrium the rate of the forward
  reaction equals the rate of the reverse reaction.
  
• Slightly soluble or insoluble salts have a
  limited solubility in water.
• Molar solubility the number of moles of salt
  that dissolve per liter of aqueous solution.

3
Exp. 29 Introduction

• A saturated solution of a slightly soluble salt
  is a result of a dynamic equilibrium between the
  solid salt and its ion in solution however,
  because the salt is only slightly soluble, the
  concentrations of the ions in solution are low.

4
Exp. 29 Introduction

• In a saturated silver sulfate solution, the
  dynamic equilibrium between the solid Ag2SO4 and
  the Ag and SO42- ion lies far to the left
  because of the low solubility of Ag2SO4.
• For this system with equation
• Ag2SO4 (s)? 2Ag(aq) SO42-(aq)
• Ksp, solubility product Ag2SO42-

5
Exp. 29 Introduction

• According to LeChâteliers principle, if you add
  an ion that is common to the salt, the
  equilibrium shifts to compensate for the added
  ions
• Ag2SO4(s) ? 2Ag(aq) SO42-(aq)
• ? ?
• 
  Na2SO4

6
Exp. 29 Introduction

• Because there are fewer moles of salt dissolved
  (in solution), the molar solubility is lower.

7
Exp. 29 Introduction

• In this experiment, you will determine the molar
  solubility of Ca(OH)2. A saturated solution of
  Ca(OH)2 has been prepared and is in equilibrium
• Ca(OH)2(s) ? Ca2(aq) 2 OH-(aq)
• You will titrate the OH- in the supernatant with
  HCl to determine OH-.

8
Exp. 29 Introduction

• Using the following equations, determine Ksp and
  the molar solubility of Ca(OH)2
• Ca(OH)2(s) ? Ca2(aq) 2 OH-(aq)
• Ca2 ½OH-
• Ksp Ca2OH-2
• Molar solubility of Ca(OH)2 Ca2

9
Exp. 29 Procedural Notes

• A.1 2 Omit the stockroom has prepared the
  saturated solutions.
• A.3 Obtain 60 mL, not 90 mL.
• B. Repeat part A with a solution of saturated
  Ca(OH)2 spiked with CaCl2.

10
Exp. 29 Report Sheet

• Questions 4 5
• This is a formal report.
• For an example (template) go to
  www.tamuk.edu/chemistry
• click on faculty/Teresa Mayorga