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Question

HCl gas is passed through an aqueous solution of 0.1 M 1-aminopropane (PrNH2) till the pH reaches 9.71. Calculate the ratio of [PrNH2]/[PrNH+3] in this solution. [Ka(PrNH+3)=1.96×1011]
log (1.96)=0.292, log (0.1)=1

A
10:1
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B
8:1
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C
4:1
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D
1:10
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Solution

The correct option is D 1:10
Given, [PrNH2]=0.1, pH=9.71, [Ka(PrNH+3)=1.96×1011]
Using the Henderson-Hasselbalch equation for weak bases:
pH=pKa+log[[conjugate base][acid]]
pH=pKa+log[[PrNH2][PrNH+3]]
pH=log Ka+log[[PrNH2][PrNH+3]]
pH+log Ka=log[[PrNH2][PrNH+3]]
9.71+log (1.96×1011)=log[[PrNH2][PrNH+3]]
9.7111+log (1.96)=log[[PrNH2][PrNH+3]]
9.7111+0.292=log[[PrNH2][PrNH+3]]
log[[PrNH2][PrNH+3]]=1 [PrNH2][PrNH+3]=0.1
[PrNH2][PrNH+3]=1:10

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