Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: clout90265 on August 22, 2008, 10:51:59 PM
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To determine the molar mass of a solid monoprotic acid, I titrated a weighted sample of the acid with standardized aqueous NaOH. why the heck did i obtain a molar mass that was too large?
i lowered it to a few errors: which one do ya think it is?
I. Failure to rinse all acid from the weighted paper into the titration vessel.
or... Addition of more water than was needed to dissolve the
acid.
or... Addition of some base beyond the equivalence point.
Thanks....this is pissing me off cause i gotta know this info for my NaOH -powered reaction RC Car!!!!!!
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If you're sure it's not a calculation error, there are a couple of things to consider.
Basically, if you're getting a molar mass that's too high for your acid then you're not having to add as much NaOH as you "should" have to.
This probably means that there is not as much acid in your titration vessel as you think there is. Most obvious is the already mentioned transfer error (not washing all of the acid into the vessel). Perhaps you could weigh out the acid sample in the vessel itself?
Another possibility is that your acid is not pure. Eg. is the acid hygroscopic and been standing on a shelf for 5 years?
Are you correctly judging the end point? At the risk of sounding ignorant about this kind of thing (sorry Borek) I do remember titrations where you observe the colour change, but then it flips back a couple of seconds later and you have to keep going until the colour change is persistant. It's been a long time, I'm no expert when it comes to titration so definately get a second opinion on that one.
Your standardised NaOH may be the issue, it may be more concentrated than you think. How is it standardised? Errors could creep in there too.
Addition of more water than needed shouldn't be a problem.
Addition of too much base would give you a molar mass for your acid that's too low.
How big is your error?