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K<sub>sp</sub> Determination

Determining Ksp

Determining the Ksp can be accomplished if you know the solubility of the slightly soluble salt. One can determine the solubility experimentally by the following procedure:

  1. Weigh an empty beaker and record
  2. measure out 100 ml of water in another beaker
  3. Add the slightly soluble salt until excess appears after stirring
  4. Decant off the solution into the pre-weighed beaker separating the excess salt
  5. Evaporate the water
  6. Weigh the beaker plus residue
  7. Subtract the empty beaker from the beaker plus residue
  8. divide the mass of residue by 100 ml to express as g/ml
  9. multiply by 1000 to get solubility in g/liter

Here is an example of determining the Ksp

What is the value of Ksp for Ag2SO4 if 5.4 g is soluble in 1L of water?

  1. Write the equilibrium equation for the dissociation

    Ag2SO4 + H2O ---> 2Ag+ + SO4-2

  2. Write the Ksp expression

    Ksp = [Ag+]2 [SO4-2]

  3. Convert the grams Ag2SO4 / liter into moles Ag2SO4 / liter

    5.4 g/l X 1 mole / MW of Ag2SO4 = moles / liter = 5.4 / 311.8 = 1.73 X 10-2 moles/liter

    Note: This mole per liter figure is how much Ag2SO4 has dissociated. According to the balanced equation in step 1 for every ONE mole per liter Ag2SO4 that dissolves (dissociates) TWO moles / liter Ag+2 ion and ONE mole/liter SO4-2 ion is produced. From the fact you should be able to do the following:

  4. Determine the [Ag+2] = 2(1.72 X 10-2)

  5. Determine the [SO4-2]

    [SO4-2]= 1.72 X 10-2

  6. Plug into the Ksp expression from step 2 above

    Ksp = [Ag+]2 [SO4-2]=

  7. Solve for Ksp

    Ksp = [3.44 X 10-2]2 [1.72 X 10-2 =

    Ksp = 11.8 X 10-4 ( 1.72 X 10-2)

    Ksp = 2.04 X 10-5

Here is an example for you to try:

The molar solubility of Lead II Iodate, Pb(IO3)2 at 26 C is 4.0 X 10-5. Determine the Ksp of Lead II Iodate.

When you get an answer you may check the correct answer

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R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

Acknowledgements:

Acknowledgements

Send Comments to R.H. Logan:

rhl7460@dcccd.edu 

All textual content copyrighted (c) 1997
R.H. Logan, Instructor of Chemistry, DCCCD
All Rights reserved

Revised: 4/12/97

The molar solubility of Lead II Iodate, Pb(IO3)2 at 26 C is 4.0 X 10-5. Determine the Ksp of Lead II Iodate.

  1. Write the dissociation equation for Pb(IO3)2

    Pb(IO3)2 + H2O = Pb+2(aq) + 2IO3-1(aq)

  2. Write the Ksp expression.

    Ksp = [Pb+2] [IO3]2

  3. Identify the eq concentration of Lead II ion

    Since according to the dissociation equation in step 1 for every Ag(IO3)2 that dissolves or dissociates one Ag+ is produced. Since the molar solubility is 4.0 X 10-5 then

    [Pb+2] = 4.0 X 10-5

  4. Determine the equilibrium concentration of Iodate ion

    For every Ag(IO3)2 that dissolves two IO3- is produced

    [IO3-] = 2(4.0X 10-5) = 8.0 X 10-5

  5. Plug in the equilibrium concentrations into expression developed in step 2

    Ksp = [Pb+2] [IO3]2

    Ksp = [4.0 X 10-5] [8.0 X 10-5]2

  6. Solve for Ksp

    Ksp = 2.56 X 10-13

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