464 ml sample of a 0336 m aqueous hypochlorous acid solution...

A 46.4 mL sample of a 0.336 M aqueous hypochlorous acid solution is titrated with a 0.446 M aqueous sodium hydroxide solution. What is the pH after 12.0 mL of base have been added? HClO is 3.5 x 10^-8. A 29.2 mL sample of a 0.496 M aqueous hypochlorous acid solution is titrated with a 0.474 M aqueous potassium hydroxide solution. What is the pH at the start of the titration, before any potassium hydroxide has been added? When a 25.3 mL sample of a 0.319 M aqueous hypochlorous acid solution is titrated with a 0.392 M aqueous barium hydroxide solution, what is the pH at the midpoint in the titration?

Transcript

00:01 First we recognize that when the base is added, the hydroxide of the base is going to react with the hypochlorous acid hclo and produce its conjugate base clo - and water.

00:16 So as long as we don't add more of the strong base than we have of the weak acid, we'll still have weak acid, but we would have made its conjugate base.

00:25 Having both a weak acid and its conjugate base in solution, we have a buffer solution, and ph can be calculated using the henderson -hasselbalch equation.

00:36 Ph will be equal to pka, which will be the negative log of the ka value, for hclo, which they gave to us as 3 .5 times 10 to the negative 8.

00:49 We then add to that the log of, some people like to do molarity of the base, hypochlorite, divided by the molarity of the acid, hclo, but in most cases, as in this case, it's easier to do a ratio of moles.

01:06 So to calculate the ph after the addition of the strong base, we simply need to calculate the moles of clo - present in solution after the addition of potassium hydroxide.

01:20 Every mole of potassium hydroxide we add creates a mole of clo-, so the moles of potassium hydroxide added, or sodium hydroxide, looks like in one location they say sodium hydroxide, in another they say potassium hydroxide.

01:43 Anyway, either way, if the answer is the same, we're adding 12 milliliters, which is 0 .012 liters, at a concentration of 0 .446 moles per liter.

02:03 This will be the moles of the hydroxide added, which will be converted into moles of clo - in a one -to -one relationship.

02:13 We then divide that by the moles of hypochlorous acid still left in solution...

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