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Posted by on Sunday, May 6, 2012 at 9:15pm.

A 0.140-mole quantity of CoCl2 is added to a liter of 1.20 M NH3 solution. What is the concentration of Co2+ ions at equilibrium? Assume the formation constant of Co(NH3)6^2+ is 5.0e31.

Please help, I don't know how to do this problem.

  • chemistry - , Sunday, May 6, 2012 at 10:25pm

    Here is how you do this with a little chicanery. First, we assume the reaction goes to completion to the right. That's not a bad assumption considering the size of the formation constant.
    .........Co^2+ + 4NH3 ==> Co(NH3)4^2+
    Initial.0.140....1.2........0
    change...-0.140.-4*1.2.....0.140
    equil.....0.......0.64......0.140

    Actually, this isn't at equilibrium since we drove it to completion. Therefore, the numbers above are a little TOO far for equilibrium; therefore, it must move back to the left. How much will it move? We set up another set of conditions but start with where we ended above.
    ..........Co^2+ + 4NH3 ==> Co(NH3)4^2+
    I.........0.......0.64......0.140
    C.........+x......+4x.......-x
    E.........x.....0.64+4x.....-x

    Substitute into K.
    K = [Co(NH3)4^2+]/[Co^2+][MH3]^4
    5E31 = [0.140-x][x][0.64+4x]^4
    Now assume 0.140-x = 0.140 and assume 0.64+4x = 0.64 and solve for x = (Co^2+). It's a very small number.

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