Chemistry
posted by Bodhi on .
A buffer is created by mixing equal concentrations of NH4+ and NH3. Calculate the volume of both solutions needed to create a buffer of pH=8.75 with a total volume of 100 mL.

Use the HendersonHasselbalch equation.
pH = pKa + log (base/acid) 
I've gotten so far, but am still unsure of how to proceed. Please correct me if I am wrong:
I know that since the concentrations of the base/acid are the same, the log(1) is 0, so the pH = pKa.
I determined the Kb by finding the Ka and using the Kw relationship.
I have a Kb value, and can determine from Kb = (OH)(A)/(HA), that Kb = (OH) since the concentrations are equal.
Now I don't know where to go from here. 
would we be able to assume that the moles are equal?