Well, there are traces of NH
4+ - which can be neglected. Let's take a closer look at the dissociation constant definition:
K
b = [NH
4+][OH
-]/[NH
3]
it can be rearranged to
[NH
4+]/[NH
3] = 10
pOH-pKbWhat for? Well, let's assume OH
- comes from NaOH only. We know pK
b to be 9.25. Now you should be able to calculate NH
4+ concentration at this pOH - if it is small enough when compared to NaOH concentration, OH
- from ammonia can be neglected.
If not - that's completely different problem. But we will deal with it ONLY if it'll be necessary