Question

Calculate the pH of a solution formed by mixing 200.0 mL of 0.30 M HOCl (hypochlorous acid) with 300.0 mL of 0.20 M NaOCl (sodium hypochlorite). The Ka for HOCl is 2.9 × 10-8 .

Answer 1

The pH of the solution is 7.54

Step-by-step explanation:

This is a buffer because we have a weak acid HOCl with its conjugate base OCl with Ka of 2.9×10⁻⁸ thus pKa = -log Ka = 7.54

therefore using Henderson Hasselbach equation: pH=pKa+log([OCl]/[HOCl]) we can determine the pH

n = mole

n of HOCl = C×V = 0.3 M ×0.2 L = 0.06 mole

n of NaOCl = 0.2 M × 0.3 L = 0.06 mole

Total volume = 300 + 200 = 500 mL = 0.5 L

final concentration: [HOCl] = 0.06/0.5 = 0.12 M; [OCl] = 0.06/0.5 = 0.12

pH=7.54+log(0.12/0.12) = 7.54

pH = pKa because the amount of HOCl = the amount of OCl in the final solution.