Additin of sodium hydroxide solution to a weak acid (HA)results in a buffer of pH 6. If ionition constant of HA is 10 - 5, the ratio of salt to acid concentration in the buffer solution will be :

Ionic Equilibrium PYQ — JEE Main 2017 · 8 Apr · Shift 1 · Q20 with Solution

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Correct answer: C

For a buffer made from a weak acid $\text{HA}$ and its salt, we use the Henderson–Hasselbalch equation:

$\text{pH} = \text{p}K_a + \log \frac{[\text{salt}]}{[\text{acid}]}$

Given:

$K_a = 10^{-5}$

So,

$\text{p}K_a = -\log(10^{-5}) = 5$

The buffer pH is given as:

$\text{pH} = 6$

Substitute into the equation:

$6 = 5 + \log \frac{[\text{salt}]}{[\text{acid}]}$

Rearranging:

$\log \frac{[\text{salt}]}{[\text{acid}]} = 1$

Therefore,

$\frac{[\text{salt}]}{[\text{acid}]} = 10^1 = 10$

Hence, the ratio of salt to acid concentration is:

$10:1$

Checking options:

A: $4:5$ ❌
B: $1:10$ ❌
C: $10:1$ ✅
D: $5:4$ ❌

So the correct option is C.

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