Slides On Solubility and Precipitation

1. From Ksp  get solubility Solubility equilibria MX2(s) ⇌ M2+(aq) +2 X−(aq), K = Ksp What is the molar solubility of CaF2? Ksp = 3.9 Five kinds of problems 10‒11 CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) 1. From Ksp  get solubility 2. From solubility  get Ksp 3. Solubility in presence of common ion 4. Will precipitation occur? 5. What remains after precipitation? 1. From Ksp  get solubility 1. From Ksp  get solubility What is the molar solubility of CaF2? Ksp = 3.9 10‒11 What is the molar solubility of CaF2? Ksp = 3.9 CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) Initial 10‒11 CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) MX2 M2+ X− excess 0 0 MX2 M2+ X− Initial excess 0 0 Change Change −x +x +2x Equilibrium Equilibrium excess x 2x Ksp = (M2+) (X−)2 = (x) (2x)2 = 4 x3 Answer: 0.00021 mol/L Check: 0.00021 (2 0.00021)2 = 3.9 × 10‒11 = Ksp 1 2. From solubility  get Ksp 2. From solubility  get Ksp The solubility of magnesium phosphate is 0.000259 g/(100 g) of water at 20 oC. Calculate its Ksp at this temperature. The solubility of magnesium phosphate is 0.000259 g/100 g of water at 20 oC. Calculate its Ksp at this temperature. Mg3(PO4)2 ⇌ 3 Mg2+(aq) + 2 PO43−(aq) Mg3(PO4)2 ⇌ 3 Mg2+(aq) + 2 PO43−(aq) Initial M3X2 M2+ X3− excess 0 0 Change Equilibrium 2. From solubility  get Ksp 3. Solubility in presence of common ion The solubility of magnesium phosphate is 0.000259 g / 100 g of water at 20 oC. Calculate its Ksp at this temperature. The molar solubility of CaF2, Ksp = 3.9 10‒11, is 0.00021 mol/L. Calculate the molar solubility in a solution of 0.015 M NaF. CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) Mg3(PO4)2 ⇌ 3 Mg2+(aq) + 2 PO43−(aq) M3X2 M2+ X3− Initial excess 0 0 Change −x +3x +2x excess 3x 2x Equilibrium 0.000259 g / 100 g  mol/L = x Ksp = (M2+)3(X3−)2 = (3 x)3(2 x)2 = 108 x5 Answer: 1.00 × 10‒23 2 3. Solubility in presence of common ion 10‒11, The molar solubility of CaF2, Ksp = 3.9 is 0.00021 mol/L. Calculate the molar solubility in a solution of 0.015 M NaF. CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) MX2 M2+ 3. Solubility in presence of common ion The molar solubility of CaF2, Ksp = 3.9 10‒11, is 0.00021 mol/L. Calculate the molar solubility in a solution of 0.015 M NaF. CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) X− Initial Initial Change Change Equilibrium Equilibrium MX2 M2+ X− excess 0 cion 3. Solubility in presence of common ion 4. Will precipitation occur? The molar solubility of CaF2, Ksp = 3.9 10‒11, is 0.00021 mol/L. Calculate the molar solubility in a solution of 0.015 M NaF. CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in a total volume of 1 L of water. Will a precipitate form? The Ksp of CaF2 is 3.9 10‒11. MX2 M2+ CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) X− Initial excess 0 cion Change −x +x +2x Equilibrium excess x cion + 2 x ≈ cion Ksp = (M2+) (X−)2 = (x) (cion)2 Large cion makes x smaller Answer: 1.7 10‒7, 0.08 % of the value in pure water! 3 4. Will precipitation occur? 4. Will precipitation occur? 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in a total volume of 1 L of water. Will a precipitate form? The Ksp of CaF2 is 3.9 10‒11. 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in a total volume of 1 L of water. Will a precipitate form? The Ksp of CaF2 is 3.9 10‒11. CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) MX2 M2+ CaF2(s) ⇌ Ca2+(aq) + 2 F−(aq) X− Initial MX2 M2+ X− 0 cM cX Initial Is (M2+) (X−)2 = (cM) (cX)2 = Qsp > Ksp? If no, then no precipitation. If yes, then a precipitate will form. Answer: Qsp = 4 10‒10 > Ksp , so CaF2(s) will precipitate 5. What remains after precipitation 5. What remains after precipitation When 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in 1 L of water, CaF2(s) precipitates. How much Ca2+ and F− remain in solution? The Ksp of CaF2 is 3.9 10‒11. When 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in 1 L of water, CaF2(s) precipitates. How much Ca2+ and F− remain in solution? The Ksp of CaF2 is 3.9 10‒11. MX2 M2+ X− Initial Revised Change Equilibrium 4 5. What remains after precipitation 5. What remains after precipitation When 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in 1 L of water, CaF2(s) precipitates. How much Ca2+ and F− remain in solution? The Ksp of CaF2 is 3.9 10‒11. When 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in 1 L of water, CaF2(s) precipitates. How much Ca2+ and F− remain in solution? The Ksp of CaF2 is 3.9 10‒11. Initial MX2 M2+ X− 0 cM cX < 2 cM MX2 M2+ X− Initial 0 cM cX < 2 cM Revised Revised excess cM − ½ cX 0 Change Change Equilibrium Equilibrium 5. What remains after precipitation 5. What remains after precipitation When 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in 1 L of water, CaF2(s) precipitates. How much Ca2+ and F− remain in solution? The Ksp of CaF2 is 3.9 10‒11. When 0.2 mmol of NaF and 10 mmol of Ca(NO3)2 are combined in 1 L of water, CaF2(s) precipitates. How much Ca2+ and F− remain in solution? The Ksp of CaF2 is 3.9 10‒11. MX2 M2+ X− MX2 M2+ X− cX < 2 cM Initial 0 cM cX < 2 cM Initial 0 cM Revised excess cM − ½ cX 0 Revised excess cM − ½ cX 0 Change −y +y +2y Change −y +y +2y Equilibrium excess cM − ½ cX + y ≈ cM − ½ cX 2y Equilibrium Ksp = (M2+) (X−)2 ≈ (cM − ½ cX ) (2 y)2 Answer: [Ca2+] = (cM − ½ cX ) = 9.9 mmol, [F−] = 2 y = 0.063 mmol Check: Qsp = (0.0099) (0.000063)2 = 3.9 × 10‒11 = Ksp 5