Question

The kb of ammonia is 1.77 ´ 10-5. the ph of a buffer prepared by combining 50.0 ml of 1.00 m ammonia and 50.0 ml of 1.00 m ammonium nitrate is __________.
a. 7.00
b. 4.63
c. 9.37
d. 4.74
e. 9.25

Answer 1

The answer is: 9.25 

according to H-H equation:

when PH = Pka +㏒[conjugate base/weak acid]

and when the concentration of ammonia = concentration of ammonium nitrate 

∴ PH = Pka 

when Ka = Kw / Kb   and Kw = 1 x 10^-14 

∴Ka = (1 x 10^-14)/(1.77 x 10^-5) 

        = 5.65 x 10^-10

∴Pka = -㏒Ka

         = -㏒(5.65 x 10^-10)

∴Pka = 9.25 

∴PH = Pka = 9.25