[Gobo]

A 20mL aliquot of an unknown, weak organic acid was titrated by a 0.1130M NaOH solution. Just 0.00213L of NaOH was needed to reach equivalence point, which was indicated by the indicator changing colour.

Given:
[NaOH]= 0.1130M
V of NaOH needed to titrate = 0.00213L
V of unknown weak organic acid = 0.025L

Find:
The initial concentration of this weak acid.

Attempt:
http://i1084.photobucket.com/albums/j409/QRAWarrior/CHMA11-IMG-001-1.png
Was it correct to use the M1V1 = M2V2 approach to get the initial concentration of the weak acid?

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Part 2: The pH of the acid was found to be 3.04.

Find:
The Ka and pKa of the weak acid.
http://i1084.photobucket.com/albums/j409/QRAWarrior/CHMA11-IMG-002.png
I did not use the pH given. What I did was that I found the molarity of NaOH after dilution into the weak organic acid solution. Then, knowing that there is a 1:1 ratio of NaOH to OH-, the [OH-]f = [NaOH]f. Final meant after dilution. Then, I used this [OH-]f to get pOH, then pH.

Then, I know that at equivalence point, pH = pKa. Then, I used pKa to find Ka.
Was it correct to use this approach to get the Ka and pKa of this acid?

[JustinCh3m]

For part 1 (I'm assuming the organic acid is monoprotic), at the endpoint moles acid = moles of base.  Molarity and Volume delivered of the NaOH allows you to calculate moles of NaOH, which equals moles of the acid.  And since you know the volume of acid...

...moles acid you now know, and you always knew L acid, so can you find the initial concentration (M) of the organic acid now?  Hope so...

[Gobo]

For part 1 (I'm assuming the organic acid is monoprotic), at the endpoint moles acid = moles of base.  Molarity and Volume delivered of the NaOH allows you to calculate moles of NaOH, which equals moles of the acid.  And since you know the volume of acid...

...moles acid you now know, and you always knew L acid, so can you find the initial concentration (M) of the organic acid now?  Hope so...

Yes it is monoprotic weak

I used
M1 = M2V2/V1 to get the concentration of the acid, but I do not know if that is the concentration they are asking for.

[Borek]

For a general outline see:

http://www.titrations.info/titration-calculation

M1V1 = M2V2 works ONLY for substances reacting 1:1, using it blindly can get you into troubles.

Numbers you used in your calculations and numbers you listed here are different, so it is hard to say if you did it correctly, or not.

Your calculation of pOH is wrong - NaOH was neutralized by the acid present, so [OH^-] doesn't equal [NaOH].