The correct option is D 1:10
Given, [PrNH2]=0.1, pH=9.71, [Ka(PrNH+3)=1.96×10−11]
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 (1.96×10−11)=log[[PrNH2][PrNH+3]]
9.71−11+log (1.96)=log[[PrNH2][PrNH+3]]
9.71−11+0.292=log[[PrNH2][PrNH+3]]
log[[PrNH2][PrNH+3]]=−1 [PrNH2][PrNH+3]=0.1
[PrNH2][PrNH+3]=1:10