[Senobim]

How many grams of KOH you have to add to 100ml 0,1M NH₄Cl solution, that pH=9,5? Kb(NH₃)=1,8*10^-5

I am not sure if you can use Henderson–Hasselbalch equation with these type of problem.
And I am not sure how to solve it properly.

[MrTeo]

I am not sure if you can use Henderson–Hasselbalch equation with these type of problem.

The approximations you use to find Henderson–Hasselbalch equations are only acceptable around the pK_b (±2 pH units) of your buffer solution. What's the pK_b?

[Senobim]

The only way i can think of is:

NH₄Cl  NH₄⁺ + Cl^-
NH₄⁺ + OH^-  NH₃+H₂O

Kb = [NH₄]*[OH^-]/[NH₃]

[NH₄]=[OH^-]
[NH₃] = ? (I have no idea)

[NH₃] = ([OH^-])²/Kb

From here I don't know how to calculate grams of KOH.

[Borek]

Have you checked if pKb is such that you can use HH equation?

What is the reaction taking place when you add KOH to NH₄⁺?

You may find some indirect hints here: http://www.chembuddy.com/?left=buffers&right=composition-calculation