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Calculating PH Concentration

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The formation constant for zinc tetraammine ion [ Zn(NH3)4+2] is 4.1 x 108. What is the concentration of free Zn+2 in a solution of 3M NH3 (total ammonia) buffered to a pH of 9.0 if the total zinc concentration is 0.1M? The Kb for NH3 is 1.8 x 10-5

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Solution Preview

For these types of multi-equilibrium questions, you need to use a few of assumptions....

First off, we see that we have total ammonia being equal to 3.0 M of total ammonia in a buffer of pH 9.0.

Using this, we set-up the buffer equation (Henderson-Hasselback somewhat) where it is:

Kb = [OH-] * [BH+]/[B]

Where Kb is the 1.8 x 10-5

[OH-] = 10^-pOH or in this case 10^-5

[BH+] is the concentration of NH4+ in our buffer system

[B] is the concentration of NH3 in our buffer system.

1.8 x 10^-5 = [10^-5] * [BH+]/[B]

Now we need algebra for the last ...

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