Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: ardyduh1manparty on May 31, 2007, 03:18:53 PM
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Ok so for the "final" in my AP chem class we were each given an unknown compound, and we are to determine what it is by doing any procedures neccessary.
So far I have determined the following about the unknown:
- White powdery/granular, crystalline solid
- Flame Test: At first it sparkles producing a lime green/yellow flame , and then it produces a steady orange/yellow flame.
- Its pretty soluble and I determined it to have a solubility of about 21.4g/100mL H2O @ 25 C
- When in a saturated solution, it forms a basic w/ pH from 9-10 (I added a few drops of phenolphthalien and the solution turned a light pink)
I was leaning towards the cation being sodium since it is soluble in water and it burns orange with the flame test, but i was thrown off by the initial lime green/yellow flame (not an accident, happens everytime i try the flame test).
Also I tried a sulfate test quickly but I didnt see a precipitate.
Anyone have any idea of what chemical I'm working with?
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Good observations. Do you know what can give green?
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barium and manganese gives off a similar green color, but they burn constantly as a green color so i doubt it has either of them as a cation.
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Could be you are left with anion... Although I must admit I don't recall magnesium giving green flame.
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No, i said manganese not magnesium. Are you saying that the cation burns lime green and once it is gone the anion burns the yellow/orange?? Do you have any other guesses tho as to what it may be?
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Manganese... my bad.
My bet is - anion green, cation yellow.
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alright thanks. so you agree that its probably sodium? hmm would you also happen to know what anoins emit a lime flame with sparks?
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Ok today i ran some more tests and I made a few key observations. First I confirmed the sulfate test, and I can safely say the compound does not contain a sulfate ion. Also I did two ammonium ion tests which both came out positive. I made a near-saturated solution of my unknown chemical, placed about 10 drops of it in a test tube then added 10 drops of Nessler's reagent, which resulted in the formation of a light yellow precipitate(some ammonia complex?). Then to back this up, in another test tube i put 5 drops of my unknown solution and then added sodium hydroxide solution, which formed a white precipitate(not sure why if it possibly contians ammonium) and I then heated the new solution and did a litmus paper test. The gas escaping turned red litmus paper blue and blue litmus paper remained blue, which shows the presence of a base, most likely ammonia. I also tested for the sulfide ion by adding cadmium nitrate solution to my unknown solution and shaking it; the solution became a little murky but I did not really see a precipitate. Finally I ran an acetate test by putting a small amount of my solid compound into a test tube then adding 10 drops of sulfuric acid and then 10 drops of ethanol; this produced a smell similar to vinegar and vodka, which would indicate the presence of an acetate ion.
So as of now I'm leaning toward the compound being ammonium acetate, although I am still confused by the flame test and the precipitation when I added NaOH solution. Let me know what you think! Thanks.
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1. Flame test doesn't fit. Try to add some ethanol, sulfuric acid, heat and check what color this solution gives.
2. Ammonium acetate solution is not basic, it is perfectly neutral, as both ammonia and acetic acid are weak to exactly the same degree.
3. White precipitate doesn't fit ammonium acetate. It could be a common ion effect if your substance contains sodium. How concentrated was NaOH?
3. Have you boiled your solution after NaOH was added, or only heated?
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Ya i just realized that its not an acetate because like you said that would make a buffer solution. I read somewhere though that ammonium compounds burn a light green color during a flame test, similar to what I observed when i burned the compound in a large quantity. As for the test with Nessler's reagent, the precipitate was yellow, which I also read indicates ammonia.
As for your questions, I believe it was 6M NaOH that I was using, and when I boiled the solution of NaOH and my unknown I thought I smelt something like ammonia but i wasn't sure, the amount of solution in the test tube was too little to get a get a good wiff of it, and it left a white coating on the test tube typical of a hydroxide solution. As for your first point, I didn't heat my solution of my unknown, sulfuric acid and ethanol; what color should it change to if ammonia is present?
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I believe it was 6M NaOH that I was using
A lot. If you have sodium salt, that could be reason of preciptation.
I didn't heat my solution of my unknown, sulfuric acid and ethanol; what color should it change to if ammonia is present?
Sorry, I wasn't clear. Check what color this solution gives in flame. It is not test for ammonia.
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Ok, so what would the color of the unknown/sulfuric acid/ethanol solution tell me? Also what would the precipitation in Nessler's reagent tell me if you think its a sodium compound? (I know Nesslers reagent is a solution of mercury(II) iodide and potassium iodide). Is the yellow precipitate i saw not an ammonium but rather a precipitation of HgI2 possibly caused by excess of iodide ion in my unknown compound?
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New observations:
-Re-tried the flame test using my unknown solution (instead of the solid) and the resulting flame was a lime green color with some sparks.
-White precipitate forms when AgNO3 solution is added to unknown solution.
-When concentrated NH4OH solution is added to the solution above, the precipitate dissappears and the solution turns clear.
-When concentrated nitric acid is added gas escapes and a white precipitate forms.
-Tested for halides by adding solid unknown into a test tube then addiding solid manganese dioxide and then sulfuric acid, and nothign happened, so no hilides were present.
-Added soduim carbonate solution to unknown solution and the solution became murky white and formed a white precipitate.
Attatched are pictures of my compound (yeah i know they are terrible quality)
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hey i like wat u have found out. I am doing the same "final" at my shcool too. I think your chemical could be Cadmium nitrate. trust me.
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Cadmium nitrate
If so, why does the pH of solution rises up?
It is possible only if the substance contains base (be it hydroxide or anion of weak acid).
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Is it possible that you have a borate? I can find very little on ammonium borate, property-wise, but a borate would explain some of the things going on. Crackling during flame test could be due to a hydrate.
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That's what my hint with ethanol & sulfuric acid was about ;) Boric acid esters burn green.
I think this method of borates determination was discussed or at least listed at chemicalforums.
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well today i tested my chemical again and I'm pretty sure the cation is barium. I did several flame tests comparing how the aqueous solution of my unknown burned to other barium solutions and they were remarkably simmilar.
So assuming its barium and considering the following observations, what do you guys suggest?:
-soluble in water (about 22g/100 mL @ 25C)
-saturated solution is basic
-does not have a sulfate, sulfide, phosphate anion or halides.
-yellow precipitate w/ Nessler's reagent
-forms white precipitate w/ 6M NaOH
-when heated it decomposed to form water and a grayish gas
-white precipitate with AgNO3 solution
I was thinking maybe barium hydroxide since that is soluble in water and basic but im not sure what gas that was when i heated it (btw it stung the nostrills)
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How do you test (chemically) for barium?
Ba(OH)2 - although strong base - is not that soluble.
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A flame test by dipping a wand into a solution of barium chloride then putting it in the flame, and the same with the unknown solution. Both burned oractically exactly the same.
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A flame test by dipping a wand into a solution of barium chloride then putting it in the flame, and the same with the unknown solution. Both burned oractically exactly the same.
This is not a chemical test. Think in terms of precipitation.
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Your results are sooooooo confusing...
Try adding a bufer so that the pH value is about 5 (acetic bufer) and add soduim sulphate. If a white precipitate forms, then you really have barium cation.
Now, about the anion: What anion can you get? Any?
It may be a posphate, so it would be possible that they gave you
Ba(NH4)PO4. Could it?
Chlorides can give the same smell as acetic anion if they are in large concentration.
The reaction with AgNO3/NH3/HNO3 giving the white precipitate, can be Cl- but could be carbonates, too. When you dissolve your solid substance in strong acid, what happens? Can you see large bubbles?
Oh, and anions CAN NOT give flame tests. They can't color the flame!
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this is a little late in comming but i ended up getting it right. It was barium acetate monohydrate. i found out that the nesslers reagent was faulty as it was contaminated with some other chemical as it was very basic when i tested the pH of it out of curiosity.