Question

The reduction potential (in volt) of a hydrogen electrode set
up with a 2 × 10⁻² M aqueous solution of a weak mono basic
acid (Kₐ = 5 × 10⁻⁵) at one atmosphere and 25°C is
(a) + 0.09 (b) + 0.18
(c) – 0.09 (d) – 0.18

Answer 1

answer : option (d) -0.018

let HA is monobasic acid.

now , HA ⇔H^+ + A^-

so, Ka = [H+][A-]/[HA]

as [H+] = [A-] = x

then, 5 × 10^-5 = x²/(2 × 10^-2)

⇒5 × 10^-5 × 2 × 10^-2 = x²

⇒10^-6 = x²

⇒x = [H+] = 10^-3

now using formula, E = E° - 0.0591/n log1/[H^+]

for monobasic , n = 1

= 0 - 0.0591/1 × log1/(10^-3)

= -0.0591 × log(10³)

= -0.0591 × 3

= -0.1773 ≈ -0.18

hence option (d) 0.18 V is correct choice.