Why Is My Calculated Molar Mass of Phthalic Acid Different from My Peers'?

[A_lilah]

Homework Statement

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.

Homework 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.

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?