Question

An industrial chemist studying bleaching and sterilizing prepares several hypoclorite buffers.

Find the pH of 0.300 M HClO and 0.350 M NaClO. (Ka for HClO = 2.9 x 10-8)pH = _______

Answer 1

pH is 7.60

Step-by-step explanation:

Let's think the equations:

HClO + H₂O ⇄ ClO⁻ + H₃O⁺

As every weak acid, we make an equilbrium

The salt is dissociated in solution

NaClO → Na⁺ + ClO⁻

HClO + H₂O ⇄ ClO⁻ + H₃O⁺

Initially 0.3m 0.35m

We have the moles of acid, and the moles of conjugate base.

Reacts x X X

Some amount has reacted, so I obtained (in equilibrium) the moles of base + that amount, and the same amount for H₃O⁺ (ratio is 1:1)

HClO + H₂O ⇄ ClO⁻ + H₃O⁺

0.3 - x 0.35 + x x

Let's make the expression for Ka

Ka = [ClO⁻] . [H₃O⁺] / [HClO]

(we don't add water, because it is included in Ka)

2.9x10⁻⁸ = (0.35+x).x / (0.3-x)

Ka is in order of 10⁻⁸, I can assume that 0.3-x is 0.3 and 0.35 +x =0.3

2.9x10⁻⁸ = (0.35)x / (0.3)

(2.9x10⁻⁸ . 0.3) /0.35 = x

2.48x10⁻⁸ = x

This is [H₃O⁺]

For pH = - log [H₃O⁺]

pH = 7.60