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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: slu1986 on November 06, 2008, 07:51:13 PM

Title: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 06, 2008, 07:51:13 PM
In my chemistry lab we did a titration of a diprotic acid<-----acid is unknown
Identifying an Unknown

I have tried everything an cannot come up with the right molecular wt for one of the unknown diprotic acids listed below:  for mol NaOH = (0.1 M) (0.00654 L) = 0.000654 mol NaOH = mol Unknown diprotic Acid   then I did 0.326 g/0.000654 mol = 498.47 g/mol <---which is too big of a number
I know you have to do something with the 2 H+ protons in the prob but I dont know what.

So if someone could please help me out and explain what to do to get the right molecular weight I would greatly appreciate it. 

here is my data:

mass of unknown = 0.326 g 
Cocentration of NaOH = 0.1000 M
Volume at 1st Equivalence Point = 6.54 ml

equation from beginning to 1st equivalence pt = H2X + NaOH ---> NaHX + H2O

List of Possible Diprotic Acids:
H2C2O4 = 90 g/mol
H2C3H2O4 = 104 g/mol
H2C4H2O4 = 116 g/mol
H2C4H4O5 = 134 g/mol
H2C4H4O6 = 150 g/mol
 
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: enahs on November 06, 2008, 10:15:14 PM
How did you determine the 1st equiv. point?
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 06, 2008, 11:14:35 PM
How did you determine the 1st equiv. point?


My teacher got it off logger pro from our data on the computer for us when we asked him how to determine the equivalence pt.  from the graph and he said let me show you an easy way to get it exactly and he had to computer give it us.  Both the pH and the volumes
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: DrCMS on November 06, 2008, 11:20:59 PM
Your maths is correct, with the data you have the acid has a molecular weight of 498g.

If the acid has to be one of the ones you listed you've got an error in the experimental part.
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: enahs on November 07, 2008, 08:30:56 AM
What was the pH of the equiv. point?

Here is an example diprotic acid titration curve:
(http://tonga.usip.edu/gmoyna/biochem341/diprotic_ac.jpeg)

Did it look like that?
Can you download the image and mark on it where you are calling the first equiv. point?

Were you given just 0.1 M NaOH, or did you verify that first (standardize it)?
Was it maybe 0.01M?
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: enahs on November 07, 2008, 09:21:05 AM
I forgot to type, what does it mean to be at an equivalence point?
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: meenu on November 07, 2008, 10:37:24 AM
Even i tried it many times so i thought i should ask this question here.
ok let me know what do u mean by

Volume at 1 equivalent point = 6.54 ml
im confused here
is this the volume of NaOH used from begening to first eqivalence point or anything else
 as for diprotic acids   , volume of NaOH used at first equivance point may be
90          ===   36.22 ml
116         ===  28.10 ml
134         ===  24.32 ml
150         ===  21.73  ml

if :) u solve it pls let me know
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: Borek on November 07, 2008, 10:42:46 AM
Your theoretical volumes are OK, problem is, 6.54 is an experimental value. It can be wrong for many reasons. Then, it can be that the acid was none of these listed (which may qualify as experimental error too  :D).
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 07, 2008, 12:31:20 PM
Your theoretical volumes are OK, problem is, 6.54 is an experimental value. It can be wrong for many reasons. Then, it can be that the acid was none of these listed (which may qualify as experimental error too  :D).

So does that mean I did everything right math wise according to solving for the molecular weight of the unknown diprotic acid? Even though there may have been an error in the experiment? I thought you had to do something with the 2 H+ protons in the calculations..I guess I'll email my teacher and see what he thinks.
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: Borek on November 07, 2008, 02:16:39 PM
So does that mean I did everything right math wise according to solving for the molecular weight of the unknown diprotic acid? Even though there may have been an error in the experiment?

Yes.

Quote
I thought you had to do something with the 2 H+ protons in the calculations.

You did - if you have used first equivalence point you have titrated the acid as monoprotic and you calculated accordingly. You will have to use stoichiometric coefficient 2 if you will titrate to the second endpoint.
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 07, 2008, 07:26:21 PM

Quote
You did - if you have used first equivalence point you have titrated the acid as monoprotic and you calculated accordingly. You will have to use stoichiometric coefficient 2 if you will titrate to the second endpoint.

Do you think that using the volume at the 2nd equivalence pt. to calculate molecular wt. would yield a better more accurate molecular weight of the unknown diprotic acid?  my 2nd equivalence pt. volume is 14.93 ml
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: enahs on November 07, 2008, 07:38:26 PM
Where on the graph are you calling the equivalence point?! Are you using the actual equivalence point, or quite commonly the pKa point by mistake!
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 07, 2008, 07:47:09 PM
Where on the graph are you calling the equivalence point?! Are you using the actual equivalence point, or quite commonly the pKa point by mistake!

Im using the Equivalence Points generated from my data and graph on logger pro.  My teacher gave me the equivalence points and the volumes in lab.  Here they are  1st equivalence point: pH = 4.29 at a vol. = 6.54 ml
2nd equivalence pt. = pH = 9.21 at a vol. = 14.93 ml
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: enahs on November 07, 2008, 07:50:53 PM
You said your teacher showed you the graph and where the equivalence points were on the graph.

Look at the figure I posted earlier. Based on what your teacher did, what is the pH of that compounds first equivalence point?

Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 07, 2008, 07:56:16 PM
You said your teacher showed you the graph and where the equivalence points were on the graph.

Look at the figure I posted earlier. Based on what your teacher did, what is the pH of that compounds first equivalence point?


The pH's that I listed are the ones for each equivalence pts.  Equiv pt 1: pH = 4.29  and Equiv pt 2: pH= 9.21 
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: enahs 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?!!!!!
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 07, 2008, 09:40:51 PM
Quote
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.   
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: enahs 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!!
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 on November 08, 2008, 01:42:59 AM
Quote
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.
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: DrCMS 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.
Title: Re: Figuring out molecular weight of an unknown diprotic acid
Post by: slu1986 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.