Calculate pH change of a basic buffer

[Krushnaraj Pandya]

Homework Statement

to 1 L solution containing 0.1 mole each of NH3 and NH4Cl, 0.05 moles of NaOH is added. The change in pH will be? (pK for NH3=4.74)

The Attempt at a Solution

I have the solution and the answer is 0.48. The following was the first step
calculated initial pH from henderson's equation, in it the concentration of salt (nh4cl) was taken 0.1 and of base (should be nh4oh here) was taken 0.1 also.
My question is, initial concentration of NH3 is given 0.1 not of Nh4Oh, why is NH3 assumed to be Nh4Oh even though it is a weak base and so doesn't completely dissociate?

 

[Borek]

Don't bother. NH₃ and NH₄OH are often used as synonyms of the same base. Both will produce OH^- on dissociation and for pH calculation it actually doesn't matter if NH₄OH exists at all.

 

[Krushnaraj Pandya]

Don't bother. NH₃ and NH₄OH are often used as synonyms of the same base. Both will produce OH^- on dissociation and for pH calculation it actually doesn't matter if NH₄OH exists at all.

alright, Thank you very much!