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Question

The dissociation constant for H2S and HS are respectively 107 and 1013. The pH of 0.1 M aqueous solution of H2S will be :

A
4
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B
5
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C
3
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D
2.5
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Solution

The correct option is A 4
Given, Ka1(H2S)=107,Ka2(HS)=1013 [H2S]=0.1 M
Since, the second acid dissociation constant is very small in comparison to the first acid dissociation constant of H2S i.e. Ka2<<<<Ka1. Thus, H+ ions are mainly obtained from the first step i.e.
H2SHS+H+[H+]=Ka1×C[H+]=107×101[H+]=108=104 MpH=log[H+]=log[104]=4

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