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Question

Calculate the concentration of NH3 and NH+4Cl present in the buffer solution of pH = 9, when total concentration of buffering reagents is 0.6 mol L1. Take pKb for NH3=4.7,log 2=0.3

A
[NH+4Cl]=0.1M and [NH3]=0.3M
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B
[NH+4Cl]=0.6M and [NH3]=0.6M
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C
[NH+4Cl]=0.8M and [NH3]=0.2M
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D
[NH+4Cl]=0.4M and [NH3]=0.2M
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Solution

The correct option is D [NH+4Cl]=0.4M and [NH3]=0.2M
Given, pKb of NH3=4.7,pH=9
total concentration of buffer = 0.6 mol L1.
Let [NH3]=b, [NH+4Cl]=a
[NH3]+[NH+4Cl]=a+b
Using the Henderson-Hasselbalch equation:
pOH=pKb+log[[conjugate acid][base]]
pOH=pKb+log[NH+4Cl][NH3]14pH=4.7+logab
14pH=4.7+logab
149=4.7+logab
149=4.7+logab
5=4.7+logab
logab=0.3
ab=2 a=2b
Given a+b=0.6; 2b+b=0.6
3b=0.6 or b=0.2 mol and a+b=0.6 mol
a+0.2=0.6a=0.4 mol
Thus, [NH+4Cl]=0.4M and [NH3]=0.2M

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