[dirtyromance]

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 K_a 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.

Please help me with this ASAP cause I can't go to bed until this is done. I'd be forever grateful.

[nj_bartel]

What do you get when you combine a strong base and a weak acid?  Has your teacher taught you about buffer solutions?  What does the Ka of lactic acid tell you about it?  Do you know about the henderson hasselback equation?

[Borek]

You don't need Henderson-Hasselbalch equation for the equivalence point pH calculation. This is not a buffer solution.

You may calculate amount of lactic acid from titration, that's simple stoichiometry. That'll give you concentration (just don't forget what the final volume is!). Then it is calculation of pH of salt of weak acid.

[dirtyromance]

You may calculate amount of lactic acid from titration, that's simple stoichiometry. That'll give you concentration (just don't forget what the final volume is!). Then it is calculation of pH of salt of weak acid.

I don't really understand how to do this at all. I don't know how to calculate the amounts from titration - he very briefly went over it and didn't say we needed to know anything about it.

[nj_bartel]

blah, sorry, borek's right.  Read over it too quickly

[dirtyromance]

blah, sorry, borek's right.  Read over it too quickly

I know that pH should be over 7. but I really don't know how to do this, at all - hence, the asking for help.

[Borek]

I don't know how to calculate the amounts from titration - he very briefly went over it and didn't say we needed to know anything about it.

Start with reaction equation - this is really very simple stoichiometry.

[AWK]

Ka for lactic acid is 1.4x10-10

Ka of lactic acid is wrong