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Topic: Need help narrowing down the possibilites of an unknown compound  (Read 12523 times)

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Offline Scyther_88

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Hello everyone. I need help narrowing down the possibilities of an unknown simple salt. These are the observations I've made:

It is a fine green powder.

It is insoluble in water. It is completely soluble in 3M HCl and 3M HNO3, but it seems the HCl dissolved it much faster.

It is insoluble in 3M and 6M NaOH, or only very slightly soluble.

It is slightly soluble in 3M NH3 and completely soluble in 6M NH3.

The anion is either: sulfide, sulfite, phosphate, or carbonate. (The lab manual stated that these were the possible water insoluble anions)

The cation is either: copper, iron, chromium, or nickel.

I'm assuming those are the possible cations because the substance is green and it did not change the color of the different solvents mentioned above. The solutions turned clear of course when the substance was dissolved within it.

I looked through the Handbook of Chemistry and Physics, but progress hasn't been great. Can anyone help me narrow down the possibilities? Thank you. 

Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #1 on: April 19, 2008, 05:17:57 PM »
Do you have other facts from your experiments that you can use, for example, when you dissolve it in acid, do you get a gas?  Does the gas have an odor.  Can you use, at least some observations, to exclude, at least some of your possible anions and cations?
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #2 on: April 19, 2008, 05:30:06 PM »
No gases or anything formed. I even heated the different solutions up and solubility did not change.

I narrowed it down already to those groups. The initial amount of possible anions are:

nitrate, chloride, bromide, iodide, sulfate, sulfide, sulfite, phosphate, carbonate

The amount of possible cations are:

copper, iron, chromium, nickel, cobalt, silver, lead, bismuth, antimony, tin, aluminum, and zinc.

Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #3 on: April 19, 2008, 05:34:12 PM »
No gases or anything formed. I even heated the different solutions up and solubility did not change.

One other anion can be excluded, in that case.
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #4 on: April 19, 2008, 05:41:41 PM »
Is it the sulfide ion?

Offline Borek

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #5 on: April 19, 2008, 05:57:20 PM »
That makes two of them :)
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #6 on: April 19, 2008, 06:12:10 PM »
Two of them? What do you mean?

Offline Borek

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #7 on: April 19, 2008, 06:13:28 PM »
Two anions can be excluded. You will smell sulfide, there is no doubt about it. And one other will be easy to spot.
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #8 on: April 19, 2008, 06:26:49 PM »
What's the other one? Sorry, I don't see it.

Offline Borek

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #9 on: April 19, 2008, 07:00:54 PM »
One of these left dissolves in acids bubbling.
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #10 on: April 19, 2008, 07:56:56 PM »
Hmmmm...bubbling. Is it the carbonate then?

Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #11 on: April 19, 2008, 09:32:27 PM »
Hmmmm...bubbling. Is it the carbonate then?

Right.  So, your list of anions can be trimmed down then?  Now, for the cations, many transition metal hydroxides are insoluble, but why do we use NH4OH and not NaOH for these sort of spot tests?
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #12 on: April 20, 2008, 10:06:09 AM »
The lab manual stated that we had to use NaOH and NH3 as the base solutions for the test.

Offline Arkcon

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #13 on: April 20, 2008, 10:30:07 AM »
The lab manual stated that we had to use NaOH and NH3 as the base solutions for the test.

OK.  What are the similarities and differences between a solution of NaOH, and a strong aqueous solution of NH3?
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Offline Scyther_88

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Re: Need help narrowing down the possibilites of an unknown compound
« Reply #14 on: April 20, 2008, 10:38:32 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- 


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