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Ksp and the Common Ion Effect

Ksp and the Common Ion Effect. April 23 2010. Ksp Defined. K sp deals with solubility (how much solute can be dissolved.) It is a version of K eq (just like K w , K a , and K b ) It says that the product (“multiplied) of all the ion concentrations in a saturated solution is a constant.

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Ksp and the Common Ion Effect

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  1. Ksp and the Common Ion Effect April 23 2010

  2. Ksp Defined • Ksp deals with solubility (how much solute can be dissolved.) • It is a version of Keq (just like Kw, Ka, and Kb) • It says that the product (“multiplied) of all the ion concentrations in a saturated solution is a constant.

  3. Example of a Saturated Solution Add a HUGE amount of NaC into a beaker of water NaCl Most will dissolve Na+ Cl- Some will remain undissolved at the bottom NaCl

  4. The saturation can change If more water is added… Some of the undissolved NaCl will now go into solution as ions Na+ Cl- Na+ Cl- NaCl NaCl

  5. The saturation can change If some water evaporates… Some of the dissolved ions will now precipitate Na+ Cl- Na+ Cl- NaCl NaCl

  6. Shown by formula: • NaCl(s) Na+(aq) + Cl-(aq) • The solution is in equilibrium so… • Keq = [Na+] [Cl-] • Note NaCl(s) is left out (it is a solid)

  7. Ksp Defined (formal) • Ksp is the Keq for a saturated solution • The solution must be saturated so a two way reaction (solid  dissolved as well as dissolved  solid) can happen • It is the product of all the dissolved ion concentrations • It measures the amount of material that can be dissolved

  8. Ksp for any salt… • AmBn(s) <==> m A+ (aq) + n B-(aq) The equilibrium constant for such a salt would be: • Ksp = [A+] m [B-]n

  9. Ksp examples • Ksp for MgSO4= [Mg2+][SO42-] • Ksp for C6H12O6 = [C6H12O6] • (note: NOT ionic) • Ksp for CaCl2 = [Ca2+][Cl1-][Cl1-] • or [Ca2+][Cl1-]2 • Ksp for Al2O3 = [Al3+] [Al3+][O2-] [O2-] [O2-] • or [Al3+]2 [O2-]3

  10. Common Ion Effect • With solutions that are saturated, addition of either ion, from another compound, will shift the equilibrium NaCl +   Na+ Cl- KCl To a saturated NaCl solution, add KCl Cl- concentration goes up Equilibrium shifts left Some NaCl will precipitate Cl- Na+ Na+ concentration goes down NaCl

  11. Common Ion Effect • When a compound is added to a solution, and that compound contains an ion in common with the solution; other ions in solution may be affected

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