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Thread: Determining the formula of an unknown acid

  1. #1 Determining the formula of an unknown acid 
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    I have to determine the formula for an unknown sample of acid (only 1 gram is provided). The methods i've come up with to determine the type of acid are the following:

    1. Melt a couple of crystals and cross-reference this value to known literature values (if anyone knows a source that provides these please post here)

    2. Titrate it using a standardized solution of sodium hydroxide and determine its molar mass. Again, cross-reference this value to literature values.

    3. Finally, use a pH probe and create a titration curve. This will give me an idea whether it is a weak/strong acid or whether it's monoprotic or polyprotic.

    If anyone knows other good analytical methods of determining the formula, please let me know. I'm also looking for some tables that contain numerous boiling points and molar masses for acids so i can compare my results and hopefully identify the acid. I've tried to use the Melting point and boiling point tables by Thomas Carnelley, but its very difficult to search for the acids since it was a PDF with images.

    Thanks.


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  3. #2  
    Forum Masters Degree organic god's Avatar
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    hmm maybe your masses are to small for this, but add a small value of your acid, to a small volume of alkali and find the heat change.
    do the same with an equal volume of a known weak acid. and a known strong acid.
    because strog acids have a higher enthalpy of neutralisation
    this can give you an indication of whether you hve a strong or weak acid aswell.
    however your titration curve idea is better.

    mass spec aswell maybe?.
    proton NMR could also tell you how many hydrogens you have on your acid.

    are you actually going to have to find its formula or is it theoretical?

    f you do can you post your results, id quite like to have a go at it


    everything is mathematical.
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  4. #3  
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    Yes i do have to actually find the formula of the unknown acid. The acid, in its solid state, is white, slightly transparent and odorless. I thought about calculating enthalpy of reactions between the acid and other solutions but since i only have 1 gram of acid and no proper calorimeter I figured it would be a waste.

    I have conducted an experiment to determine the melting point and it seems that the salt melts at temperatures greater than 240 degrees Celsius. The thermometer I used only allowed a reading up to 250 degrees Celsius so i stopped at 240 to prevent it from exploding. I was not expecting such a huge melting point, so this should be able to filter out most carboxylic acids as well as most common acids (and all acids i can think of :? ).
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  5. #4  
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    hmm lets think...

    that high temperature.

    could indicate an amino acid. in solid form they have high melting points due to electrostatic attractions between zwitterions.

    so i say our best bet is to look at amino acids.

    in which case we need to melt it and note 3 things.

    what temperature we notice discolouration (white to brown)
    what temperature any liquid is produced
    what temperature any gas bubbles are produced.

    we can then look this up against a table and see what we get
    everything is mathematical.
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  6. #5  
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    Quote Originally Posted by organic god
    hmm lets think...

    that high temperature.

    could indicate an amino acid. in solid form they have high melting points due to electrostatic attractions between zwitterions.

    so i say our best bet is to look at amino acids.

    in which case we need to melt it and note 3 things.

    what temperature we notice discolouration (white to brown)
    what temperature any liquid is produced
    what temperature any gas bubbles are produced.

    we can then look this up against a table and see what we get
    Yeah that would be ideal, however the highest temperature i can get a reading of is 250 degrees Celsius. I thought of finding the rate of change of temperature and assume it remains constant throughout the heating (i'm using a hot plate and an aluminum block that holds the thermometer + sample). Since the heat produced by the hotplate will eventually stabilize, the assumption that temperature increases at a constant rate will hold true, so i will be able to find the temperature after a given time interval.

    Tomorrow, i will titrate three samples of my acid and find its molecular mass, then we can go from there.

    Thanks for the help so far
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  7. #6  
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    maybe the 250 gives us a clue, amino acid higher mpt than 250 that could narrow down the list of suspects.

    i'd imagine it to be quite a common AA if its an assignment youve beenset

    molecular mass would be a useful confirmation, il search some values for you.

    also can you produce a titration curve for us. and work out the Pka values, that would be helpful
    http://www.chem.ubc.ca/courseware/pH/aadata.html
    everything is mathematical.
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  8. #7  
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    Why not just titrate like you said in #2? That seems much easier.
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  9. #8  
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    because from the titration all we would have is the molecular mass and that is all.
    everything is mathematical.
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  10. #9  
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    I've titrated it today but i need to first produce a titration curve so i know if the acid is mono/di/polyprotic so i can get the ratios right. I will also calculate this from the titration curve and compare the two molecular masses obtained. Lastly, i will find the pKa value and hopefully i will be able to identify the acid. I discussed with my teacher and i asked him whether or not it's an amino acid and he said he can't even remember (and was too lazy to check his list) but it's not likely that it is one. However, it's good not to completely ignore the amino acids because he's a little tricky and likes to confuse me (as well as my classmates). Are there any other acids that have higher boiling points besides amino acids? I've looked at the melting points of carboxylic acids and none of them even come close to 250. I'm gonna try and react it with an alcohol and see if it produces an ester just to be completely sure it's not a carboxylic acid. But the only type of bond that is likely to be strong enough not to be broken at 250 degrees is ionic. I doubt any hydrogen bond would even come close to that even for larger molecules.

    I calculated that the molecular mass (assuming it's monoprotic) is around 265 g/mol. This is the minimum mass, indicating that it's a very heavy molecule. None of the standard amino acids have molecular masses as large as 265 g/mol so it must be something else. I'm going to use the Aldrich catalogue to find some heavy acids.
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  11. #10  
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    i was sure it was an amino acid, but if it's not i dont know what it can be.

    a mass spec would be useful if you could get one
    everything is mathematical.
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  12. #11  
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    I've actually made a mistake. I've performed a titration using a pH probe, and it's molar mass is 57.77 g/mol. It's also a weak acid with a pKa value of about 9.1-9.3. I've also investigated its melting point, and it seems to decompose once the temperature reaches 100 degrees. It does in fact melt and even evaporates at a temperature of 220 degrees. If heated from room temperature to 240 degrees, it decomposes leaving white solid behind, which does not melt. I've also performed an esterification and it seemed to form an ester when i reacted it with methanol. I've also done a flame test and it burns with a green colour. I thought it could contain copper but that would not agree with the compound's molar mass. I've been trying to find possible matches, but i haven't found any compound that satisfies any two of the conditions i have found.

    It is also a monoprotic acid.
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