# Calculating PH Concentration

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 ...

#### Solution Summary

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Calculating Ion Concentration and pH

1) Calculate the hydrogen ion concentration and the pH of the solution formed when 0.020 gram-formula weight of nitric acid is added to one liter of aqueous 0.20 M ammonia solution. Assume no significant volume change upon addition of the acid.

2) Calculate the molar concentration of acetic acid at equilibrium in an acetic acid solution that is 1.1% ionized.

3) Given a pure 0.50 M of HCN solution, calculate the volume that when diluted to a total volume of 70.0mL will yield a pH that is 0.65 unit higher than that of the given solution. Consider the volumes of the original solution and of the diluent to be precisely additive.

4) Calculate the pH of 0.0013 M of NaCN.

5) Calculate the pH of a solution which is 0.092 M in acetic acid and 0.092 M in sodium acetate.

6) Calculate the pH of a mixture of 50mL of 0.10 M ammonium hydroxide and 51mL of 0.10 M of HCl.

Please see attached.

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