Correct Answer: 3

Explanation

Step 1: Convert given data

pKa = 4 ⇒ Ka = 10⁻⁴

Concentration of HA = 10 mM = 0.01 M

Step 2: Write expression for Ka

Ka = [H⁺][A⁻] / [HA]

Let degree of dissociation be x.

[H⁺] = x , [A⁻] = x , [HA] ≈ 0.01

(because degree of dissociation is negligible)

Step 3: Substitute values

10⁻⁴ = x² / 0.01

x² = 10⁻⁶

x = 10⁻³

Thus,

[H⁺] = 10⁻³ M

Step 4: Calculate pH

pH = −log[H⁺] = −log(10⁻³) = 3

Therefore, the pH of the solution is 3.

Related JEE Main Questions