A 75.0-mL volume of 0.200 M NH3 (Kb=1.8×10−5) is titrated with 0.500 M HNO3. Calculate the pH after the addition of 13.0mL of HNO3.

Question

cuanchi · Answer

First you have to do a stoichiometry calculation to see how many moles of base (NH3) you still have and how much moles of salt NH4NO3 you has formed. 

NH3 + HNO3 -> NH4NO3


I estimate by the concentrations and the volumes you will be in the region of a buffer 
you have to use the Henderson Hasselbalch  equation:

pH = pKa + log[Base]/[Acid]

in this case you will have to convert the Kb to Ka
Kw=Kb x Ka
the base will be the left over moles of NH3, and the acid the moles of the salt

cuanchi · Answer

here a better explanation https://answers.yahoo.com/question/index?qid=20080321231140AAkuF7T