Identifying Unknown Compounds in a Lab

[a.dunn.wa]

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

 

[dx/dy=?]

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 ;)

 

[dx/dy=?]

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!

 

[chemisttree]

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.

 

[LudusRex]

Hello Andy,

Identifying unknown compounds in a lab can be a challenging and exciting task. Your resources seem quite limited, but there are still several tests you can perform to help identify the compounds. Here is a rough procedure that I would suggest:

1. Start by observing the physical properties of the unknown compounds, such as color, odor, and solubility in water and ethanol. This can give you an initial idea of what type of compounds you are dealing with.

2. Use the red and blue litmus papers to test for acidity/basicity. This will help you determine if the compounds are acids or bases.

3. Perform a conductivity test using the conductivity meter. This will tell you if the compounds are ionic or covalent in nature.

4. Use the thermometer and hot plate to determine the melting and boiling points of the compounds. This can give you further clues about their identity.

5. Use the silver nitrate solution to test for halides (chloride, bromide, iodide). This will involve adding a few drops of the solution to a small amount of the unknown compound and observing for a color change.

6. Use the bromothymol blue, phenolphtalein, and phenol red indicators to test for the presence of acidic or basic functional groups.

7. Use the 0.1 M NaOH and HCl solutions to perform a neutralization test. This will help you determine if the compounds are acidic or basic in nature.

8. Use the 0.1 M K2SO4 solution to test for the presence of sulfate ions. This can be done by adding a few drops of the solution to a small amount of the unknown compound and observing for a white precipitate.

9. Use the 0.1 M H2SO4 solution to test for the presence of carbonate ions. This can be done by adding a few drops of the solution to a small amount of the unknown compound and observing for the formation of bubbles.

10. Finally, use the Bunsen burner to perform a flame test. This involves heating a small amount of the unknown compound in the flame and observing the color of the flame. Different compounds produce different colors, which can help you narrow down your options.

As for the chemicals with asterisks, here is a brief explanation of their functions:

- Silver nitrate solution: used to test for halides (chloride, bromide, iodide).
- K2SO4 solution