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.
[(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...)