HClO is a weak acid (Ka = 4.0 × 10–8) the salt NaClO acts as a weak base. What is the pH of a solution that is 0.046 M in NaClO at 25 °C?

NaClO is a soluble ionic compound which completely dissolves in water to give Na⁺ and ClO⁻ ion.

ClO⁻ then dissociate in water with dissociation constant Kb :
ClO⁻(aq) + H₂O(l) ⇌ HClO(aq) + OH⁻(aq) .... Kb
where Kb = [HClO][OH⁻]/[ClO⁻] = ([H⁺][OH⁻]) / ([H⁺][ClO⁻]/[HClO]) = Kw/Ka

Before dissociation :
[ClO⁻]ₒ = 0.046 M
[HClO]ₒ = [OH⁻]ₒ = 0 M

At equilibrium :
[ClO⁻] = (0.046 - y) M ≈ 0.046 M, assuming that 0.046 M ≫ y M
[HClO] = [OH⁻] = y M

Kb = Kw/Ka = [HClO][OH⁻]/[ClO⁻]
(1 × 10⁻¹⁴)/(4.0 × 10⁻⁸) = y²/0.046
y = 1.07 × 10⁻⁴
[OH⁻] = 1.07 × 10⁻⁴

pOH = -log[OH⁻] = -log(1.07 × 10⁻⁴) = 3.97
pH = 14 - 3.97 = 10.03