[azmanam]

All this is fantastic for your write up.  The problem solving skills you're picking up here are awesome.

sodium benzoate. NaOH + Benzoic acid also produces sodium benzoate.

Close.  But benzoic acid's MW is 122 and it's boiling point is 249.  There's error bars on all this data but to say 140 is close enough to 249 is a bit of a stretch...  Benzoic acid is not the acid I have in mind.  I like what you're thinking though.

Here's the problem.  I don't think you can trust your pH data at all.  As soon as the NaOH acted on your compound, 2 new compounds were formed - both acids and both with their own pK_as.  So now you have 3 compounds in solution: the original compound and the two derivatives.

Do you still have time to do more tests? 

[azmanam]

What were the exact instructions for this assignment?  And what techniques have you learned about in class so far?  that is, what techniques does your instructor assume you have reasonable access to?

[r4bbit_]

No   no more tests. But wow, NaOH produced 2 new derivatives of my compound? Now I am just beaten  .Hmmm, I will do some more research, just out of curiousity, does your guess have double carboxyl groups?

[r4bbit_]

The instruction was "Kids, each of you will get an acid sample, tell me what it is" I am currently in a special program kinda of like Gifted program, I'm currently in grade 12 but I have a reasonable understanding of years 1-2 uni organic chemistry, equilibrium, rates of reaction and etc.

[azmanam]

does your guess have double carboxyl groups

yes.  but be careful with how you integrate that.  I don't know where you're going with it, but make sure you check your guesses for MW, mp, bp, etc.

Do you have any new guesses?

[r4bbit_]

If the acid is dicarboxylic, then wouldn't it be diprotic? That wouldn't go along with the titration curve right?

[r4bbit_]

My guess was malic acid btw.

[azmanam]

Right, which is what I was warning you about.  The acid is not diprotic.

What might have a second carboxyl group - not be a second acid - but react with NaOH to produce two new compounds, both of which are themselves acids?

[azmanam]

And the original starting material will produce an ester with ethanol.  How did you run that test, btw?

[r4bbit_]

For the esterification, I placed about 0.05 g of unknown acid sample into a beaker, then dissolved with i blieve 5 mL of H2O and then poured 5 mL of ethanol. Added about 5 drops of concentrated H2SO4 and maintained it in a hot bath at 60'C for 15 minutes. Smelled it after 15 minutes.

"second carboxyl group - not be a second acid - but react with NaOH to produce two new compounds, both of which are themselves acids"

Wow, I am getting speechless now  . Would that NaOH be reacting in elimination mechanism then?

[azmanam]

Smelled it after 15 minutes.

did it smell like nail polish remover?  Did you need 2 molar equivalents to get the reaction to go to completion?  What if I told you that was the same mechanism that was ruining your pK_a experiment?

Ok.  Let's recap - tell me what information we now know.

[azmanam]

did you notice a smell after your pK_a experiment, too?

[r4bbit_]

Yup, it smelled like nail polish remover.

So far this is what I know for sure:

mp,bp
Melting point around 135'C for sure
Boiling point around 145'C for sure
These were done in a fumehood and also controlled fairly well so no contaminations could've messed up the results.

mw
Molar mass, empirical mass is 68 g/mol but for the purpose of this investigation, I can see that it is likely not 68 g/mol because the volume of NaOH added might've halved to drive another reaction as you mentioned.

organic + esterfies
reacts with ethanol to produce an ester that smells like nail polish remover, and it also burns above the flame.

pka
I found it to be averaging around 4.4 but it is not very credible in this case as another reaction could've happened and i was measuring simply the neutralization of another acid.

[r4bbit_]

Did not smell after Pka experiment.

[azmanam]

Alright, I can't give you any more information you didn't observe experimentally.  Your MW data and pK_a data are unfortunately useless from here on out.  If you factor in the NaOH side reaction, you can make sense of the MW data, but it won't help you figure out the original acid.  We can talk about that later, though.  For now, the whole pK_a experiment should essentially be discarded.

Take a look at the information you have, plus the information we've worked to here and see if you and integrate all that into a couple plausible structures.  It's past my bedtime, but someone else can probably pick it up - and I'll check back in the morning.  Don't give yourself a headache.

You're doing good work, keep it up