Explanation

A basic buffer is formed by a weak base and its conjugate acid (salt). Here, NH₄OH is a weak base and NH₄Cl is formed after partial neutralization with HCl.

For a basic buffer:

$$ \text{pOH} = \text{p}K_b + \log\frac{[\text{salt}]}{[\text{base}]} $$

Given pH = 9.25

$$ \text{pOH} = 14 - 9.25 = 4.75 $$

Since pK_b = 4.75,

$$ 4.75 = 4.75 + \log\frac{[\text{salt}]}{[\text{base}]} $$

$$ \log\frac{[\text{salt}]}{[\text{base}]} = 0 $$

$$ \frac{[\text{salt}]}{[\text{base}]} = 1 $$

So, moles of NH₄Cl formed must be equal to moles of NH₄OH left after reaction.

Option B:

Moles of NH₄OH = 0.2 × 0.5 = 0.10 mol Moles of HCl = 0.1 × 0.5 = 0.05 mol

After reaction:

NH₄OH left = 0.10 − 0.05 = 0.05 mol NH₄Cl formed = 0.05 mol

Salt : Base = 1 : 1 → required pH = 9.25

Hence, option B is correct.

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