JEE Main 2026 — Jan 22, Shift 2 — Chemistry solution

JEE Main2026Jan 22, Shift 2Chemistry

Which of the following mixture gives a buffer solution with pH = 9.25 ?

Given : $p K_{b} (N H_{4} O H) = 4.75$

A
0.2M $N H_{4} O H$ (0.4 L) + 0.1M HCl (1L)
B
0.2M $N H_{4} O H$ (0.5 L) + 0.1M HCl (0.5 L)
C
0.5M $N H_{4} O H$ (0.2 L) + 0.2M HCl (0.5 L)
D
0.4M $N H_{4} O H$ (1 L) + 0.1M HCl (1L)

Solution

For $p H = 9.25$ , $pO H = 14 - 9.25 = 4.75 = p K_{b}$ . From Henderson:

$pO H = p K_{b} + lo g \frac{[ Salt ]}{[ Base ]}$

So $lo g \frac{[ Salt ]}{[ Base ]} = 0 \Rightarrow [Salt] = [Base]$ (equal millimoles of $N H_{4} C l$ and unreacted $N H_{4} O H$ ).

Checking option (2): millimoles $N H_{4} O H = 0.2 \times 500 = 100$ ; millimoles $H C l = 0.1 \times 500 = 50$ . After reaction: $N H_{4} C l = 50$ , leftover $N H_{4} O H = 50$ — equal. ✓

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