Camille ([info]dirtyromance) wrote in [info]chemistryhelp,
@ 2008-05-26 05:04:00
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Acids & Bases & Equilibrium - oh my. :(

I need help with this question.

A 35.0mL sample of (monoprotic) lactic acid, C3H6O3, is titrated with 20.0mL of a 4.0x10-4 mol/L sodium hyroxide solution. What is the pH of the resulting solution at the equivalence point, if Ka for lactic acid is 1.4x10-10?

I don't know how to even really approach this since our teacher didn't tell us what to do if there is a concentration & an amount used... it's just never happened before. And, I don't know how to do this without the concentration of lactic acid... both those things combined leave me a confused girl.



But, this is what I have so far:

C3H6O3 + NaOH --> NaC3H5O3 + H2O

Kc = [NaC3H5O3] [H2O] / [C3H6O3] [NaOH]

...but I really don't think any of that is correct.

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[info]uberjason
2008-05-26 03:58 pm UTC (link)
It looks like you're supposed to assume that the result of this titration leaves you exactly at the equivalence point. Then, since you're given the concentration and amount of NaOH, you can calculate the amount (and then the concentration) of lactic acid. Now you can proceed as usual.

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