Question

How many grams of dry NH4Cl need to be added to 2.10L of a 0.700M solution of ammonia, NH3, to prepare a buffer solution that has a pH of 8.59? Kb for ammonia is 1.8×10−5.

Answer 1

I got 9.25527 for the pKa
then for the Henderson-Hasselbalch equation:
9.00=9.25527+log(.600/acid)
10^(-.25527)=(.600/acid)correct to this line. Then
0.5556 = 0.6/acid and
acid = 0.6/0.5556 = 1.08 which is the reciprocal of your number)

.5556/.600M=acid
acid=.925925...M
(.925925)*2.10L= Molarity