the pH of 0.04M hydrazine solution is 9.7.Calculate ionization constant kb and PKb

Asked by  | 4th Feb, 2012, 03:50: PM

Expert Answer:

The reaction:
 
NH2NH2  +  H2O ------>  NH2NH3  +   OH–
 
From   the  given pH   the Hion concentration can be measured.
 
So, we have:[H+] = antilog (–pH) = antilog (–9.7) = 1.67 × 10–10
 
Now, [OH–]  = Kw / [H+] = 1 × 10–14 / 1.67 × 10–10                           
= 5.98 × 10–5
 
The concentration of the corresponding hydrazinium ion is also the same as that of hydroxyl ion. The concentration of both these ions is very small so the concentration of the undissociated base can be taken equal to 0.004M.
 
Thus, Kb = [NH2NH3+][OH–] / [NH2NH2]= (5.98 × 10–5)2 / 0.004 = 8.96 × 10–7
 
So, pKb = –logKb = –log(8.96 × 10–7) = 6.04.

Answered by  | 5th Feb, 2012, 10:21: AM

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