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Solution of 0.1 N NH4OH and 0.1 N NH4Cl has pH 9.25. Then find out pKb of NH4OH.

1. 9.25
2. 4.75
3. 3.75
4. 8.25

Correct answer: 4.75

Solution

The given solution is a buffer of NH4OH and NH4Cl. Using the Henderson-Hasselbalch equation for a basic buffer, pH = pKb + log([Salt]/[Base]). Since [Salt] = [Base], log([Salt]/[Base]) = 0, so pH = pKb. Thus, pKb = 9.25.

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