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Question

Find the pH of 1 L of a buffer solution containing 0.01 M NH4Cl and 0.05 M NH4OH at 25C
The base dissociation constant for NH4OH is 1.8×105
Take log(1.8)=0.26

A
9.96
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B
10.36
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C
11.56
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D
12.56
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Solution

The correct option is A 9.96
This is a basic buffer formed by a weak base (NH4OH) and its salt (NH4Cl) with strong acid (HCl)
pKb=log(Kb)pKb=log(1.8×105)pKb=(50.26)=4.74
pOH=pKb+log[salt][base]pOH=pKb+log([NH4Cl][NH4OH])pOH=4.74+log(0.010.05)pOH=4.74+log(0.2)=(4.740.7)pOH=4.04pH+pOH=14pH=14pOH=144.04=9.96

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