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Topic: Need help narrowing down the possibilites of an unknown compound  (Read 12516 times)

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Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #15 on: April 20, 2008, 10:57:27 AM »
Well, I'm not too sure. Let's see...

NaOH is a strong base. NH3 is a weak base. Both are clear in solution form.

NaOH + H2O --> Na+  +  OH-

NH3 + H2O --> <-- NH4+  +  OH- 

Right.  Both form the hydroxide ion, which precipitates most transition metals.  But, your sample dissolved in ammonia, why is that?  Hint:  I call it ammonia here, and not just a weak base.

Listen, either you've heard of this before, or you come across it in your text book, or you won't know it.  There are limited hints we can give you for these qualitative analysis labs that we all have to slog through.  The lab writeup may be explaning it for you, or it may be in your text, or you were expected to know it already.  But try to see is you can figure it out.
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #16 on: April 20, 2008, 11:10:17 AM »
Oh, wait. It dissolved in NH3 because the cation formed a complex ion with NH3 ? Which cation is able to do that?

Yeah, this lab is really annoying. I just have to narrow down my cations and anions so I have a direction of what to test next week in lab. I don't want to trudge through a million different tests randomly. Besides, I don't have the time to do that either.

Thank you for all the help you are giving me.

Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #17 on: April 20, 2008, 11:24:16 AM »
Oh, wait. It dissolved in NH3 because the cation formed a complex ion with NH3 ? Which cation is able to do that?

Yes.  Two cations from your abbreviated list can, I've done them myself.  One can't -- it behaves exactly the same way with an excess of ammonium hydroxide as with an excess of sodium hydroxide, again, tried it, and saw it.  And one, unfortunately, I haven't done, so I can't be sure.  Either one of us can look it up, and it might as well be you. ;)

Yeah, this is, in some ways the beginnings of chemical engineering, where you use chemical concepts, to build a plan or flow chart to reach an ending conclusion.  No one memorizes all these reactions.  But is is, ultimately, about asking the right questions, narrowing down your search, and searching for what you didn't know.

So.  Can we have a further abbreviated list given what we've determined so far?  And maybe you'd like to work out a flow chart, one that separates the anions and cations with each test.  That's probably a good way to summarize your work for the lab report.
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #18 on: April 20, 2008, 12:16:45 PM »
Oh no, I just realized something else.

In my lab manual it listed the colors of ions in solution. I choose copper, iron, chromium, and nickel as my cations because they are under the green category. Remember, the unknown substance is a green powder.

However, when it completely dissolved in the HCl, NO3, and 6M NH3, the solution was colorless. It was not a green solution or any other color. But of course when the substance did not dissolve in the H2O and NaOH, the substance remained green in the solution and gave the solution a green tint before collecting at the bottom of the test tube.

Should I be looking at the cations listed under "colorless in solution" rather than the green ones? If so, that would really suck because the possible cations behave that way are: silver, lead, bismuth, antimony 3 and 5, tin 2 and 4, aluminum, and zinc. I really hope this isn't the case.

Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #19 on: April 20, 2008, 12:30:30 PM »
It's confusing though. Usually when there is a change of color, it indicates a new solid formed or something. My green powder substance simply dissolved and the solution become colorless. I centrifuged it too and no ppt formed.

Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #20 on: April 20, 2008, 12:46:08 PM »
From your observations, I'm thinking you made a very dilute solution.  For example, the green salts of copper, nickel, or even iron, produce a green solution.  The ammonium complex, of at least some of these, produce vibrant colors, that might not be green, depending on the cation. 

You list no such observations, and the the only reason I can think for why is that your solutions are very dilute.  I'm kinda ignoring your color information, because it's suspect in this regard.  But, if you poke around, and see the actual colors of these salts, and their solutions, and work from your memory, you may be able to pull some info you can use to help your identification.
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #21 on: April 20, 2008, 04:02:33 PM »
Yeah, the solutions were very dilute. I added a tiny amount, about 1/3 or even less of a microspatula amount into 2mL (about) solvent.

So you think I either have Nickel, Copper, or Iron? Is the green salt a dead giveaway?

Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #22 on: April 20, 2008, 05:55:12 PM »
*Ignore me, I am impatient*.

Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #23 on: April 20, 2008, 05:57:57 PM »
If you poke around online, typing in the names of possible salts, you may find a picture that matches the particular shade of green that you saw in your bulk unknown.  I have known green salts of copper, nickel and even iron.  Here's a freebie, copper sulfide, if you look it up online, you'll know, is black.  And I've known yellow, or rust-red, salts of iron.  I don't think I've ever seen chromium as a cation.  So picking this bit of info out of a mountain of data is up to you.

And there's no need to *Ignore me, I am impatient*.  Only you'll recognize a photo of the salts and what you saw in the lab.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #24 on: April 20, 2008, 06:34:26 PM »
Yeah, I looked through the handbook of physics and chemistry and browsed through the color descriptions. The closest ones I found were nickel phosphate and copper phosphate. The iron and chromium ones did not have the initial green powder description.

My best guess at this point is that I have either nickel or copper for my cation.
Either phosphate or sulfite for my anion.

Here's an interesting point. Copper phosphate is insoluble in water and soluble in acids. It is also soluble in NH4OH. Since it is soluble in NH4OH, does it mean it is also soluble in NH3? If so, this is the closest match I can find so far.

Offline Borek

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #25 on: April 21, 2008, 03:27:58 AM »
NH4OH and NH3(aq) are synonyms. As far as I remember it is not clear whether dissolved ammonia reacts with water and becomes NH4OH or not.
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