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chem please help meeeeeeeeeee?

1. What is the pH of a saturated solution of Mg(OH)2 at 25oC? For Mg(OH)2, Ksp=7.1x10-12.

2.

a. [Cu2+] remaining in 444 mL of a solution that was originally 0.742 M CuSO4, after passage of 2.61 A for 261 s and the deposition of Cu at the cathode.

b.The time required to reduce [Ag+] in 153 mL of AgNO3(aq) from 0.192 to 0.121 M by electrolyzing the solution between Pt electrodes with a current of 4.72 A.

3 Answers

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  • 1 decade ago
    Favorite Answer

    Ksp = [Mg++]*[OH-]^2

    let [OH-] =X then [Mg++] = 0.5X (1 to 2 mole ratio)

    7.1E-12 = 0.5X * X^2 = 0.5X^3

    X = 2.42E-4

    p(OH) = 3.616

    pH = 10.38

  • Anonymous
    4 years ago

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  • ?
    Lv 6
    1 decade ago

    Let X = moles per liter of Mg++ that goes into solution

    Let 2X = moles per liter of OH- that goes into solution

    The equilibrium reaction is Mg(OH)2(s) at equilib with Mg++(aq) plus 2OH-(aq)

    Ksp = 7.1 x l0^-12 = (X)(2X)^2 or (Mg++)(OH-)^2

    7.1 x l0^-12 = 4X^3

    X^3 = 7.l x l0^-12 over 4 which equals. l.77 x l0^-12

    and X = l.9 x l0^4 so 2X which is the concentration of OH- = 3.84 x l0^-4

    :pOH = -log (OH-) or -log (3.84 x l0^-4) which = 3.42

    and pH = l4-pOH or l4-3.42 which = 10.58

    2. 2.61 Amps times 261 seconds = 681.2 coulombs.

    One mole of electrons carries a charge of 96,485 coulombs.

    So we have passed 681.2 over 96485 or .0071 moles of electrons through the solution which will plate out only half this many moles of Cu++ ions due to the fact that each ion has a charge of plus 2, so we are plating out .0071 moles over 2 which = .0035 moles Cu++ plated out or reduced to Cu metal.

    The original number of moles of Cu++ in the solution = Molarity times volume in liters.

    .742 Molar times .444 Liters = .329 moles original Cu++ in solution so Cu++ remaining would be .329 moles original minus .0035 moles plated out or ,326 moles Cu++ still in solution.

    b. Moles Ag+ reduced = .l92 molar minus .121 molar or .07l moles Ag+ reduced which would also require .071 moles of electrons

    Coulombs required = 96485 coulombs per mole e- times .071 moles e- = 6850.4 coulombs.

    And if you are plating out at 4.72 Amps, this means that you are passing 4.72 coulombs of charge per second.l

    So to get seconds, divide total coulombs passed, 6850.4 by 4.72 coulombs per second which =

    1451.4 seconds.

    Source(s): retired chem teacher.
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