Determination of the Solubility Product K sp of Ca(OH) 2 Experiment 13

#12 Determination of the Solubility Product K sp of Ca(OH) 2 Goal: Find the amount of OH - in a saturated Ca(OH) 2 solution to calculate K sp of Ca(OH) 2 Method: Prepare saturated Ca(OH) 2 solution from Ca metal Titrate Ca(OH) 2 solution with HCl Calculate K sp from M OH-

Saturated Solution Visible solid with solution above Ca(OH) 2(s) Ca 2+ (aq) + 2 OH - (aq)

Solubility Equilibrium Molecular view: Ions dissolve Ions re-precipitate At dynamic equilibrium: rate dissolution = rate re-precipitation Constant ion concentration

Solubility Product, K sp Ionic compound of M n+ and X n- : Solubility product constant, K sp : degree of solubility (how much dissolves?) concentration of dissociated ions at saturation

Range of Solubilities Large K sp :higher solubilitymore dissolved ions Small K sp :lower solubilityfewer dissolved ions

Ca(OH) 2 and K sp from stoichiometry: [Ca 2+ ] = ½ [OH - ] Ca 2+ 2OH - Ca(OH) 2

Strong base-strong acid titrations At equivalence point: mol H + = mol OH - n H+ present = n OH- added pH Vol. HCl added

Data 1)Find mol OH - in filtrate 2) Find M OH - 3) Find K sp : individual, average,  → report on board 4) Find class K sp,  5) Compare to K sp, literature = 6.5×10 -6

END OF THE EXPERIMENT

Example Titration Data TrialDrop of OH-M HClDrops of HCl

Overview Procedure: Prepare saturated Ca(OH) 2 solution Collect filtered solution (filtrate) Titrate aliquots to equivalence point with HCl Data Analysis: Calculate n OH- titrated from n H+ used Calculate [OH - ] in filtrate Calculate K sp = ½ [OH - ] 3 (individual, average, class,  )

Part 1 Preparation of saturated Ca(OH) 2 1) Add Ca metal to water Saturated when solid Ca(OH) 2 remains Add Ca (s)

Part 1 Preparation of saturated Ca(OH) 2 2) Filter the solution DO NOT WASH (why?) 3) Stopper solution clear cloudy

Part 1 Preparation of saturated Ca(OH) 2 4) Calculate approximate [OH - ] K sp, literature = 6.5 × 10 -6

Part 2 Titrate OH - saturated with HCl  Record M HCl M HCl,conc  Calculate HCl molarity to use with 10 mL filtrate M HCl,dilute  Calculate HCl dilution Dilution to use ~12-15 factor mL HCl per trial OH - HCl titrant

Example Dilution Data From K sp Desired volume In 10.00mL Approx. volume Molarity needed On bottle Volume of conc Final volume Dilute molarity ~mL needed

Part 2 Titrate OH - saturated with HCl  Aliquot+100 mL H 2 O+BCP V HCl,dilute purple  yellow  3 – 4 trials (3 good)  Calculate actual M OH - M OH-  Calculate K sp K sp individual, average, class average OH - HCl titrant

Data 1)Find mol OH - in filtrate 2) Find M OH - 3) Find K sp : individual, average,  → report on board 4) Find class K sp,  5) Compare to K sp, literature = 6.5×10 -6

Example Titration Data

Report Abstract Results including  n OH-  [OH - ]  your  K sp , your  Ksp, class  K sp , class  Ksp Sample calculations  n OH-  [OH - ]  K sp  Error in your  K sp  and class  K sp  Discussion/review questions

Strong base-strong acid titrations At equivalence point: mol H + = mol OH - n H+ present = n OH- added pH Vol. HCl added