Problem: Calculate the pH of a 0.10-M CoCl 3 solution. The Ka value for Co(H2O)63+ is 1.0 X 10 -5.

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We are being asked to determine the pH of 0.10 M CoCl3


CoCl3 dissociates in water in the following manner

  • CoCl3 -> Co3+ + 3 Cl-
  • Cl is neutral

Co is hydrated and forms Co(H2O)63+ which releases H+

Co(H2O)63+ + H2O -> Co(H2O)5(OH)2+ + H3O+ Ka = 1.0 X 10 -5


From this, we can construct an ICE table. Remember that liquids are ignored in the ICE table.



The Ka expression for Co(H2O)63+ is:

Ka =productsreactants = [H3O+][COH2O5OH2+][COH2O63+]


Liquids are ignored. Note that each concentration is raised by the stoichiometric coefficient: [Co(H2O)63+], [H3O+] and [Co(H2O)5(OH)2+] are raised to 1.


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Problem Details

Calculate the pH of a 0.10-M CoCl 3 solution. The Ka value for Co(H2O)63+ is 1.0 X 10 -5.

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