Find the pH of 1L of a buffer solution containing 0.01MNH4Cl and 0.05MNH4OH at 25∘C The base dissociation constant for NH4OH is 1.8×10−5 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(1.8×10−5)pKb=(5−0.26)=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)=(4.74−0.7)pOH=4.04pH+pOH=14pH=14−pOH=14−4.04=9.96