organic compound molar mass

A_lilah

1. The problem statement, all variables and given/known data

We did an experiment in lab today where we were given a compound and we had to figure out the chemical formula and molar mass using titration (titrated with NaOH). The compound I got was phthalic acid, and the average molar mass I got was ~135g. This is about 30 grams off of what everyone else in my group got.


2. Relevant equations

[(M NaOH)(L NaOH)]/2 = moles of phthalic acid (there were two equivalency points, which is why it had to be divided by 2)

moles phthalic acid / mass of sample used = grams of phthalic acid

We were then given the mass percents of each element, and I divided these by the atomic masses of each element, then used the ratio of them to figure out the chemical formula, and whether it was a multiple of the ratios used or not (it was: actual formula = C6H4O2)

My professor said all of my calculations were correct.

3. The attempt at a solution

So my ultimate question is: Why is my molecular mass 30 grams short of everyone else who used phthalic acid? I did the titration 4 times (the 4th time with the TA watching~ an interesting experience), and all 4 times I got between 130 and 137.

Any ideas would be great. Thanks!! :)

(ps~ this may be the wrong place to post this, but I wasn't sure...)

queenofbabes

If even your professor has verified all your calculations, then perhaps you can blame a contaminated chemical sample @.@

A_lilah

I thought of that too, but the prof said that the samples were all from the same source... good thinking though

Borek

Show details of your work.

Have you used the same titrant others did?

chemisttree

Which volume of NaOH titratant did you use? The one for the first eq. point or the second?