Question

The pH of0.004 M hydrazine is 9.7 . Calculate it's ionisation constant Kb and Pkb.

Answer 1

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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^-10Now, [OH-] = Kw / [H+] = 1 × 10^-14/1.67 × 10^-10 = 5.98 × 10^-5.The concentration of the corresponding hydrazine 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 0.004 MThus, 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.