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.

Question

Chemistry

Brett Nicholson

1 July, 12:39

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.

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Volvo · Answer 1

The pH of the solution is 7.54

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.