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Topic: Figuring out molecular weight of an unknown diprotic acid  (Read 40783 times)

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Offline slu1986

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Figuring out molecular weight of an unknown diprotic acid
« 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
 

Offline enahs

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #1 on: November 06, 2008, 10:15:14 PM »
How did you determine the 1st equiv. point?

Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #2 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

Offline DrCMS

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #3 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.

Offline enahs

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #4 on: November 07, 2008, 08:30:56 AM »
What was the pH of the equiv. point?

Here is an example diprotic acid titration curve:


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?
« Last Edit: November 07, 2008, 08:56:36 AM by enahs »

Offline enahs

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #5 on: November 07, 2008, 09:21:05 AM »
I forgot to type, what does it mean to be at an equivalence point?

Offline meenu

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #6 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

Offline Borek

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #7 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).
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #8 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.

Offline Borek

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #9 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.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #10 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

Offline enahs

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #11 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!

Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #12 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

Offline enahs

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #13 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?


Offline slu1986

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Re: Figuring out molecular weight of an unknown diprotic acid
« Reply #14 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 

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