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Topic: Figuring out molecular weight of an unknown diprotic acid  (Read 40781 times)

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Offline enahs

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #15 on: November 07, 2008, 09:20:33 PM »
Great! Are you sure?

Again, on the graph I posted earlier; where is the equivalence point at based on what your teacher told you?!!!!!

Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #16 on: November 07, 2008, 09:40:51 PM »
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Again, on the graph I posted earlier; where is the equivalence point at based on what your teacher told you?!!!!!

Im sorry but I don't understand what u mean about how to explain where the points are on your graph...just use my equivalence pt data that I gave u and compare it to your graph.  The data that my teacher gave me is the data from my experiment not his... b/c we were asking how to find the equiv pt. when we had our data on the computer in the lab along with our graph and he did our equiv pts. w/ logger pro and said they were right.. ??? I am so confused..I need to write a lab report on this, but I don't want to write it with the wrong answers.   

Offline enahs

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #17 on: November 07, 2008, 10:19:21 PM »
!!!

You said he showed you how to determine the equivalence point from the graph!
So, now, go look at the graph I posted. From how he showed you how to graphically determine the equivalence, on the graph I posted, what would you call the equivalence point!!!!!!!! I have asked it 5,000 times now. I do not know how else to ask it! The graph I posted is not exactly your graph! Just, on the graph I posted, if you were to determine the equivalence point graphically as your teacher told you, where would it be????!


It is not that complicated, I am just trying to find out if you know where the equivalence point on the graph I posted is, and if that corresponds to the real equivalence point; and thus if your "equivalence" point in your experiment is  the equivalence point, or something else!!

Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #18 on: November 08, 2008, 01:42:59 AM »
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You said he showed you how to determine the equivalence point from the graph!

I said he got the numbers off logger pro..He didn't show us how to get the equivalence pt from the graph! He did it for us, by using the computer.  Then he gave us our values..and that's why im here.

Offline DrCMS

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #19 on: November 08, 2008, 08:49:59 AM »
What enahs is trying to get you to understand is how to spot the equivalence point. 

We DO NOT care what your teacher or computer said the equivalence point was we want you to look at the graph enahs posted and tell us what it means.  If you can not do that there is no point in you writing up your lab report because you do not understand what you have been doing.  If you blindly follow what the computer tells you the equivalence point is but can not spot if it has got it right then you have learnt nothing.  Sometimes the electronic systems pick the wrong point and you have to manually change it.  If you do not understand it you cant do that.  If you can not do that then what use are you?  We'd just use trained monkey to do the work not chemists with degree's.

As has been pointed out by many people already based on the data you have given us the acid has a molecular weight of 498.  You've said that was not one of the possibilities so we are trying to find out where the error is.  One of the obvious sources for error is the endpoint which is why enahs has been asking you the question over and over.

That said your data is crap. 
1st equivalence point: pH = 4.29 at a vol. = 6.54 ml  give a molecular weight of 498
2nd equivalence pt. = pH = 9.21 at a vol. = 14.93 ml gives a molecular weight of 436
 the difference between the 1st and 2nd point of 8.39ml gives a molecular weight of 389
 
Can you see why we are questioning your equivalence points.

Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #20 on: November 08, 2008, 01:48:47 PM »
What enahs is trying to get you to understand is how to spot the equivalence point. 

We DO NOT care what your teacher or computer said the equivalence point was we want you to look at the graph enahs posted and tell us what it means.  If you can not do that there is no point in you writing up your lab report because you do not understand what you have been doing.  If you blindly follow what the computer tells you the equivalence point is but can not spot if it has got it right then you have learnt nothing.  Sometimes the electronic systems pick the wrong point and you have to manually change it.  If you do not understand it you cant do that.  If you can not do that then what use are you?  We'd just use trained monkey to do the work not chemists with degree's.

As has been pointed out by many people already based on the data you have given us the acid has a molecular weight of 498.  You've said that was not one of the possibilities so we are trying to find out where the error is.  One of the obvious sources for error is the endpoint which is why enahs has been asking you the question over and over.

That said your data is crap. 
1st equivalence point: pH = 4.29 at a vol. = 6.54 ml  give a molecular weight of 498
2nd equivalence pt. = pH = 9.21 at a vol. = 14.93 ml gives a molecular weight of 436
 the difference between the 1st and 2nd point of 8.39ml gives a molecular weight of 389
 
Can you see why we are questioning your equivalence points.

I understand all that....It's not necessary to be rude in your explanation though. I know how to find the equivalence point from enah's graph b/c it is a perfect snake shape and the equivalene points are obvious..Equiv pt 1 = pH = 5.5 and OH- equivalents = 1 and Equiv pt. 2 = pH = 12.5 and OH- equivalents = 2...However on my graph It is difficult and nearly impossible to locate the equivalence pts b/c it isnt in a perfect snake shape like enah's and it's just a million dots..Im guessing that's why he had the computer do it.  I took his word when he gave us the equiv pts and volume b/c he is a chemistry lab professor and thats what he does.  I am not getting a degree in chemistry...I am a Kinesiology major sooo therefore Inorganic and Organic Chemistry is required in my major. 

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