Question

Use Goal Seek in Excel to calculate [OCl–], [HOCl], [H ], and [OH–] for a 0.0482 M NaOCl solution. The pKa for HOCl is 7.53. Also determine the fraction of OCl- that is hydrolyzed. Ignore activity coefficients.

Answer 1

[OCl^-] = 0.0482 M

According to Henderson-Hasselbulch equation:

pH = 7 + 1/2 (pKa + Log[NaOCl])

= 7 + 1/2 (7.53 + Log0.0482)

= 10.11

Now, [H⁺] = 10^-pH = 10^-10.11 = 7.8*10^-11 M

i.e. [OH^-] = 10^-14/[H⁺] = 10^-14/7.8*10^-11 = 1.3*10^-4 M

Therefore, [HOCl] = 1.3*10^-4 M

The hydrolysis constant, Kh = Kw/Ka = 10^-14/10^-7.53 = 10^-6.47

Now, the degree of hydrolysis (h) = (kh/[NaOCl])^1/2 = (10^-6.47/0.0482)^1/2 = 0.00265

Therefore, the percentage of OCl- hydrolyzed = 0.00265*100 = 0.265%