Question

1) Kb for NH3 is 1.8x10^-5. What is the pOH of a .20M aqueous solution of NH4Cl at 25 degrees Celsius? (Please show work)

2) Determine the pH of a 0.15 M aqueous solution of KF. For hydrofluoric acid, Ka=7.0x10^-4. (Please show work)

3) An aqueous solution of _____ will produce a neutral solution.

A) NaNO2

B) LiNO3

C) KF

D) Rb2CO3

E) NH4NO3

Answer 1

1) Ka of Nh4+ = Kw/kb = 10^-14 /1.8x10^-5 = 5.55 x 10^-10

NH4+ (aq) <---> Nh3 (aq) + H+ (aq) ,

at equilibrium [NH4+]= 0.2-X , [NH3] = [H+] = X

Ka = [Nh3] [H+] /[NH4+]

5.55 x 10^-10 = ( X) ( X) / ( 0.2-X)

X= [H+] =1.054 x 10^-5   , pH = -log [H+] = -log ( 1.054x10^-5) = 5

pOH = 14-pH = 14-5 = 9

2) F- (aq) + H2O (l) <--> HF (aq) + OH- (aq) , Kb = Kw/KaHf = 10^-14 / 7x10^-4 = 1.4286 x 10^-11

at equilibrium [F-] = 0.15-X , [HF] =[OH-] = X

Kb = [HF][OH-] /[F-]

1.4286 x 10^-11 = ( X) ( X) / ( 0.15-X)

X = [OH-] = 1.464 x 10^-6 , pOH = -log ( 1.464 x 10^-6) = 5.835

pH = 14-5.835= 8.165

3) storng acid and strong base combination gives salt which is neutral , hence its LiNO3 , since LioH is strong base and HNO3 is strong acid