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UNIT III Tutorial 11: Predicting Precipitates and Maximum Ion Concentration

UNIT III Tutorial 11: Predicting Precipitates and Maximum Ion Concentration. Objectives. In this Tutorial you will be shown: 1. How to tell if a precipitate will form when two solutions of known concentration are mixed.

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UNIT III Tutorial 11: Predicting Precipitates and Maximum Ion Concentration

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  1. UNIT IIITutorial 11: Predicting Precipitates and Maximum Ion Concentration

  2. Objectives In this Tutorial you will be shown: 1. How to tell if a precipitate will form when two solutions of known concentration are mixed. 2. How to tell if a precipitate will form when a given mass of a solid is added to a solution. 3. How to calculate the maximum possible concentration of an ion in solution, given the concentration of another ion and the Ksp . 

  3. Predicting Precipitates The solubility table tells you when precipitates form, right? NOT ALWAYS! It tells you which combinations of ions have a solubility of > 0.1 M! If the table, for example, predicts a precipitate between Ca2+ ions and SO42- ions, if we mix the ions in low enough concentrations, a precipitate might not form.

  4. Predicting Precipitates The quantity which determines the concentrations of ions allowed before a precipitate will form is Ksp ! Ksp = [Ca2+] [CO32-] = 5.0 x 10-9 This means, that in a saturated solution (just on the verge of significant precipitation)… 5.0 x 10-9 = [Ca2+] [CO32-] This is the most the product of the concentrations of these two ions can be. If we put any more Ca2+ or CO32- in at this point, the solution can't hold any more, and a precipitate will form. 

  5. Predicting Precipitates • When Two Solutions are Mixed Example 1 We mix solutions of Ca2+ and CO32-, so that [Ca2+] = 2.3 x 10-4 M and the [CO32-] = 8.8 x 10-2 M. The Ksp for CaCO3 is 5.0x 10-9. Will a precipitate form?

  6. Predicting Precipitates When Trial Ksp > Ksp , a precipitate will form. When Trial Ksp = Ksp, the solution is saturated and is on the verge of precipitating. When Trial Ksp < Ksp , the solution is unsaturated and a precipitate will NOT form.

  7. Predicting Precipitates Example 2 When 50.0 mL of 0.0035 M Ca(NO3)2 solution is mixed with 150. mL of 2.0 x 10-5 M Na2CO3 solution, will a precipitate form?

  8. Predicting Precipitates Example 3 250.0 mL of 3.0 x 10-4 M Ba(NO3)2 is mixed with 350.0 mL of 0.0020 M Na2SO4 solution. Will a precipitate form?

  9. Predicting Precipitates B) When a Solid is Added to a Solution Example 4 Will a precipitate form if 2.6 grams of K2CO3 is added to 200.0 mL of a 2.0 x 10-3 M solution of Mg(NO3)2?

  10. Predicting Precipitates Example 5 Will a precipitate form if 3.8 grams of Ca(NO3)2 is added to 250.0 mL of 0.0050 M Na2SO4 solution? Calculate the Trial Ksp first.

  11. Maximum Ion Concentration Imagine a saturated solution of AgCl. Once the solution is saturated (at equilibrium), there is a limited number of Ag+ and Cl- ions which can exist dissolved in the solution. What do you think tells us this limit? The Ksp !

  12. Maximum Ion Concentration The Ksp for AgCl is 1.8 x 10-10. The Ksp expression is: Ksp = [Ag+][Cl-] What this tells us is that: The maximum product of the concentrations: [Ag+][Cl-] cannot exceed 1.8 x 10-10.

  13. Maximum Ion Concentration If we added some Ag+ or Cl- ions, the product [Ag+][Cl-]would be higher than 1.8 x 10-10 for a very short time. (At this point it would not be at equilibrium any more!) But what would happen then is the excess Ag+ or Cl- ions in solution would precipitate onto the crystal (solid), until the product [Ag+][Cl-] in the solution was again equal to 1.8 x 10-10, and equilibrium was again established.

  14. Maximum Ion Concentration Example 6 Calculate the maximum possible [I-] in a solution in which [Ag+] is 1.0 x 10-5 M.

  15. Maximum Ion Concentration Now, looking at the results of this problem: •  If [I-] in this solution is less than 8.5 x 10-12 M, the solution will be unsaturated. •  If the [I-] is just equal to 8.5 x 10-12 M, the solution will be saturated. •  If more [I-] is added, if will react with some Ag+ and some AgI(s) precipitate will form.

  16. Maximum Ion Concentration Example 7 Calculate the maximum possible [Cl-] in a solution in which [Pb2+] is 1.0 x 10-3 M.

  17. Maximum Ion Concentration Example 8 Calculate the [Pb2+] necessary to just start the precipitation of PbBr2 from a solution in which the [Br-] is 0.0030 M.

  18. Maximum Ion Concentration Example 9 A waste solution from a certain industrial process contains 0.050 M Cu2+ ions. In order to collect the Cu2+ from the solution for recycling, some OH- ions are added to produce a precipitate of Cu(OH)2. OH- ions are added until the excess [OH-] is 2.0 x 10-3 M. Calculate the [Cu2+] remaining in the solution. The Ksp for Cu(OH)2 is 1.6 x 10-19.

  19. Maximum Ion Concentration Example 10 5.0 litres of tap water has a [Ca2+] of 0.0060 M. Calculate the maximum mass of sodium carbonate (Na2CO3) which can be added without forming a precipitate of CaCO3.

  20. Maximum Ion Concentration Example 11 Calculate the mass of NaI which must be added to 500.0 mL of a 2.0 x 10-4 M solution of Pb(NO3)2 in order to form a saturated solution of PbI2 .

  21. Which Precipitate Forms First Sometimes a solution is added to another solution with a mixture of ions. You may be asked to determine which precipitate will form first. Example 12 0.010 M NaI is added dropwise to a solution containing 1.0 M Ag+ and 1.0 M Cu+. Which precipitate will form first?

  22. Which Precipitate Forms First Example 13 If 0.1 M KCl is added dropwise to a beaker containing 0.10 M Ag+ and 0.10 M Pb2+, which precipitate will form first? Show all equations and calculations.

  23. Assignment • Hebden Textbook p. 98-99 Questions #56-69

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