A student is asked to determine the value of Ka for hypochlorous acid by titrati
ID: 499357 • Letter: A
Question
A student is asked to determine the value of Ka for hypochlorous acid by titration with potassium hydroxide.
The student begins titrating a 48.5 mL sample of a 0.384 M aqueous solution of hypochlorous acid with a 0.269 M aqueous potassium hydroxide solution, but runs out of standardized base before reaching the equivalence point. The student's last observation is that when 24.5 milliliters of potassium hydroxide have been added, the pH is 7.235.
What is Ka for hypochlorous acid based on the student's data?
Explanation / Answer
Concentration of acid = 0.384 M
volume = 48.5 ml
moles of acid = concentration*volume = 0.384*48.5 = 18.6 mmol
concentration of base = 0.269 M
volume = 24.5 ml
moles of base = 0.269*24.5 = 6.6 mmol
Using Handerson Hasselbach equation,
pH = pKa + log [base]/[acid]
7.235 = pKa + log (6.6/18.6)
7.235 = pKa + log 0.355
7.235 = pKa - 0.45
pKa = 7.685
pKa = -log Ka
7.685 = -log Ka
Ka = 2.1*10-8
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