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Question

The increasing order of pKb value of the following is:
I. HCC II.NH2 III.CH3

A
I < III < II
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B
I < II < III
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C
II < III < I
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D
III < II < I
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Solution

The correct option is D III < II < I
Convert conjugate base into acid by adding H, find the acidic character and then reverse the order after removing H ions that would be the order for basic character.

HCCH is most acidic because of the presence of sp hybridized C as acidity is directly proportional to % of s-character. In between N and C, N is more electronegative than C. So, acidity of NH3>CH4.

Acidity: HCCH>NH3>CH4

Basicity: HCC<NH2<CH3

The lower the value of pKb, the stronger is the base.
pKb order:
III < II < I

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