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Salt Solubility

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Selective precipitation. Will a precipitate form, given the ion concentrations? What concentrations of ions are needed to bring about precipitation? ... – PowerPoint PPT presentation

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Title: Salt Solubility


1
Salt Solubility
  • Chapter 18

2
Solubility product constant
  • Ksp
  • Unitless
  • CaF2(s) ? Ca2(aq) 2F-(aq)
  • Ksp Ca2F-2
  • Ca2 molar solubility (in M)
  • Get Ksp values from Appendix J, pages A-24-25

3
We have problems
  • Ksp for BaSO4 1.1 x 10-10 _at_ 25C
  • Calculate this salts solubility in water in g/L.

4
Solution
5
Problem
  • If Ca2 2.4 x 10-4 M, what is the Ksp value
    of CaF2?

6
Solution
7
Solubility and the Common Ion Effect
  • Problem How much AgCl (g/L) would dissolve in
    water given Ksp 1.8 x 10-10?

8
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9
Solubility and the Common Ion Effect
  • If solid AgCl is placed in 1.00L of 0.55M NaCl,
    what mass of AgCl (g/L) would dissolve?
  • Before solving this, would you expect it to be
    lesser or greater than in pure water?
  • Think back to Le Châteliers Principle and the pH
    of buffers

10
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11
Basic anions and salt solubility
  • Do we recall what a conjugate acid/conjugate base
    is?
  • If salt has conjugate base of weak acid, then
    salt more soluble than value given by Ksp
  • Why?
  • PbS(s) ? Pb2(aq) S2-(aq)
  • S2-(aq) H2O(l) ? HS-(aq) OH-(aq)
  • Where net Ksp ? Kb (gtKsp)

12
Effect of pH on solubility
  • Mg(OH)2 (s) ? Mg2(aq) 2OH-(aq)
  • If one adds base, OH-, which way will the
    equilibrium shift?
  • Will more solid form or dissolve?
  • If one adds acid, H3O, which way will the
    equilibrium shift?
  • Will more solid form or dissolve?

13
Effect of pH on solubility
  • Mg3(PO4)2 (s) ? 3Mg2(aq) 2PO4-3(aq)
  • If one adds base, OH-, which way will the
    equilibrium shift?
  • Will more solid form or dissolve?
  • If one adds acid, H3O, which way will the
    equilibrium shift?
  • Will more solid form or dissolve?

14
Effect of pH on solubility
  • MgCl2 (s) ? Mg2(aq) 2Cl-(aq)
  • If one adds base, OH-, which way will the
    equilibrium shift?
  • Will more solid form or dissolve?
  • If one adds acid, H3O, which way will the
    equilibrium shift?
  • Will more solid form or dissolve?

15
So what can we say about the conjugate base and
solubility?
16
The reaction quotient, Q
  • Q tells us
  • Whether its at equilibrium
  • If not, which way itll shift
  • CaF2(s) ? Ca2(aq) 2F-(aq)
  • Ksp Ca2F-2
  • Q Ca2F-2
  • If Q Ksp, then _at_ eq. soln is saturated
  • If Q lt Ksp, then will shift to right (dissolve
    more) soln is unsaturated
  • If Q gt Ksp, then will shift to left (not dissolve
    anymore, ppt out) soln is supersaturated (will
    ppt out)

17
Check out the heating pads video
  • http//genchem.chem.wisc.edu/demonstrations/Gen_Ch
    em_Pages/11solutionspage/crystallization_from_supe
    r.htm

18
Problem
  • AgCl placed in water. After a certain amount of
    time, concentration 1.2 x 10-5 M.
  • Has the system reached eq.?
  • If not, will more solid dissolve?
  • How much more?
  • (Ksp 1.8 x 10-10)

19
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20
Selective precipitation
  • Will a precipitate form, given the ion
    concentrations?
  • What concentrations of ions are needed to bring
    about precipitation?
  • Basically, compare Q to Ksp

21
Problem
  • If Mg2 1.5x10-6 M, and enough hydroxide ions
    are added to make the solution 1.0 x 10-4 M of
    hydroxide ions, will Mg(OH)2 precipitate?
  • If not, will it occur if the concentration of
    hydroxide ions is increased to 1.0 x 10-2 M?
  • Ksp 5.6 x 10-12

22
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23
Complex ion equilibria
  • Complex ion a central metal ion (e- acceptor
    Lewis acid) bound to 1 or more ligands
  • Ligand neutral molecule or ion that acts as
    Lewis base (e- donor) with the central metal ion
  • Kf values on page A-26, Appendix K

24
Example
  • What is Knet for the reaction below?

25
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26
Problem
  • What is the net rxn and value of Knet for
    dissolving AgBr in a solution containing S2O32-?
  • Ksp 5.4 x 10-13
  • Kf 2.0 x 1013

27
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