JEE Chemistry 2026 Question with Solution

Given: A weak base $B$ has $pK_b = 5.699$. $x$ mL of $0.02$ M HCl and $y$ mL of $0.02$ M weak base are mixed to form $100$ mL buffer of pH $9$.

Find: The values of $x$ and $y$, and hence the correct option.

For a basic buffer, first convert $pK_b$ into $pK_a$ of the conjugate acid $BH^+$.

Now apply the Henderson–Hasselbalch equation for the buffer $B/BH^+$:

Using $\log 5 = 0.699$,

Moles of HCl added are:

Moles of base added are:

After neutralisation, all added HCl converts an equal amount of $B$ into $BH^+$. Therefore,

So the buffer ratio becomes:

Since both are prepared from solutions of the same molarity, the common factor $0.02/1000$ cancels:

Also, total volume is $100$ mL:

Therefore, $x = 14.3$ mL and $y = 85.7$ mL. The correct option is D.

Given: Both HCl and weak base $B$ have concentration $0.02$ M.

Find: Why $\dfrac{y-x}{x} = 5$ can be written directly.

The base buffer contains weak base $B$ and its conjugate acid $BH^+$ formed by partial neutralisation:

Initial moles:

After reaction:

Hence,

Since the required ratio is $5$,

This directly gives the same result used in the standard solution.