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Identifying Unknown Compounds in a Lab

  1. May 31, 2012 #1
    Hello,

    I am looking for assistance from you chemistry maestros, geniuses, and experts on how to handle a 5-day lab of identifying unknown compounds using limited resources.

    We were given four unknown compounds and a list of fifty possibilities. I am not as interested as in you guys helping me identify them (atleast not yet), but more interested on how you guys would tackle this assignment--no procedure is given to us, we can do whatever we want given the resources we are provided with.

    Resources given are as follows:
    Water
    Ethanol
    Red and blue litmus paper
    0.1 M silver nitrate solution*
    Bromothymol blue indicator
    Phenolphtalein indicator
    0.1 M NaOH solution
    0.1 M K2SO4 solution*
    phenol red indicator
    0.1 M HCl
    0.1 M H2SO4

    And then stuff like hot plates, B.B.s, elec. conductivity meters, thermometers etc. are also provided.

    So basically I have two questions:
    1.) What would your rough procedure look like and why (what each test would accomplish)

    and

    2.) What do all of these given testing chemicals do? I know what some do but the ones with asterisks I put next to I am not really sure of, and I hope you can help me out.


    I appreciate your guy's help so much, and I hope you take the time to reply if you can help. It means a lot to me!

    Andy
     
  2. jcsd
  3. Jun 2, 2012 #2
    I would use chomatography (including GC), mass-spec and IR-spec to determine probable compounds.
    If these are not available, looks like a fair bit of thinking will be needed ;)
     
  4. Jun 2, 2012 #3
    Further to the above-
    Both of the asterix'd compounds are salts.. hint...

    Silver Nitrate is commonly used to extract halides.
    Potassium Sulfate is commonly used as a disintergrating agent.

    Good luck!
     
  5. Jun 2, 2012 #4

    chemisttree

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    You need to develop a flowsheet to test your unknowns. They all start with, "Is the unknown soluble in water?" If yes, you test pH. If not you continue, "Is the unknown soluble in ethanol?" The point is for you to learn the properties of the 50 possible unknowns and develop a flowsheet to test all of the possibilities. Then go into the lab and execute your flowsheet. Simple, huh? This is an example of a portion of the flowsheet you will need to develop.

    Well, let's say your unknown was calcium carbonate. Try to dissolve .1 g into 1-2 mL water. It fails. Try to dissolve it in ethanol. That fails too. We know we have an insoluble salt or an organic compound that is insoluble in both water and ethanol. Look at the list of 50 possibilities and eliminate all that don't fit the results. Divide the slurry in water into two roughly equal portions. To one portion of the slurry of the unknown in water add a roughly equal volume of 0.1 NaOH. No change. To the other portion of the slurry of the unknown in water add a roughly equal volume of 0.1 HCl. Slurry fizzes and goes into solution. We have an insoluble carbonate but is it MgCO3 or CaCO3? Neutralize the solution by adding a drop of phenolphthalein indicator followed by just enough NaOH to either just form a ppt or the solution turns faintly pink. If the solution starts to form a ppt, centrifuge and decant the supernatant. Test the supernatant with a few drops of K2SO4. It forms a ppt so it must be CaCO3. If it were MgCO3, the MgCl2 solution would not form a ppt with the added SO4-2 since MgSO4 is much more soluble than CaSO4.
     
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