Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Scyther_88 on April 19, 2008, 05:03:39 PM
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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.
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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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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.
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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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Is it the sulfide ion?
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That makes two of them :)
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Two of them? What do you mean?
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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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What's the other one? Sorry, I don't see it.
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One of these left dissolves in acids bubbling.
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Hmmmm...bubbling. Is it the carbonate then?
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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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The lab manual stated that we had to use NaOH and NH3 as the base solutions for the test.
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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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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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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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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.
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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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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.
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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.
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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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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?
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*Ignore me, I am impatient*.
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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.
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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.
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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.