Separation lab

Hello,

I have a chemistry question and was hoping you could help me out.

Okay heres the basics:

There are 7 possible chemicals all of these chemicals are XNO3

PbNO3  ZnNO3 AgNO3 CuNO3 ect.

Out of these 7 possible chemicals I will be given either 1, 2 or 3 of them in a vile (I dont know how many I will have or what chemicals are in my vile.)

I have to design an experiment that will separate the chemicals in my vile and ultimately I need to be able to say which chemicals these are.

I'm not really sure how to start this or how to research it, could you point me in the right direction.  If you need any more details, like the other 3 chemicals (haha) I'll be sure to get back to you just let me know.
-BC

Hi BC!

Well, first I'd take a good look at the solubility rules. I'm surprised by how many students aren't taught the solubility rules.

You can find them here:  solubility rules

Keep in mind that for each salt, you have a cation (positive ion) and an anion (negative ion.) The solubility rules tells you which salts are soluble in water. Remember that when a salt is soluble, it falls apart, separating into the cation and anion, and "disappears" in the water, because the ions are so small, they get surrounded by water molecules, and of course they are too small to be seen with your eyes. When they are NOT soluble, they stick together, form a clump, and you can see the precipitate.

All salts of nitrate (NO₃^-) are soluble, so all of these in salts in the XNO3 form will fall apart in water into X+?(magnitude of charge unknown but positive) and NO₃^-.

So what you have in solution are one or more unknown positive ions, and the nitrate ion, which you can ignore, since it won't stick to anything, according to the solubility rules. (When an ion sticks to another they form a solid and precipitate.)

Then you can try combining the positive ions with, say, a salt containing Cl-. (like NaCl.) All salts of Na+ are soluble, so the Cl- and Na+ will separate from each other in solution, and the Cl- will stick to some positive ions but not others. It will stick to lead, silver, and mercury cations, according to the rules. So if you get a precipitate you know you have lead, silver, or mercury in there.

Of course, save some solution ahead of time before you do this! So you can test the rest of it in other ways.

If would have been helpful if you could tell me the three other chemicals, and the level of your class. If the other salts contain group 2 cations (Ca Ba or Sr) you can distinguish them by their inability to form a precipitate with OH- ion (add NaOH or KOH as a source of OH-)

Now, if you want to distinguish between lead and silver, since both preciptiate with Cl-, you could try precipitating them with an iodide containing salt instead (like NaI). Both the resultant AgI and PbI2 are yellowish, but PbI₂ will turn red when you heat it. You might want to look up other salt colors on the internet.

You could try to form metal ion complexes with ammonia. Transition metals form complexes with ammonia, that render the solution colored, and the color depends on the ion present. You can take a look at this site

Or search on "complex ions" "metals" and "colors"

Now, I don't know how advanced your class is, but if you need to separate lead and silver in the same container, you can get into the nitty gritty of solubility product constants, symbolized as Ksp.

It turns out that solubility rules sort of gloss over the fact that salts that are called insoluble are still just a bit soluble, really, if they are dilute enough.

The solubility product constant (Ksp) is a number, an equilibrium constant, if you have had that (?). Without going into the details of how it is calculated, (I will if you ask), the smaller the Ksp number, the less soluble the salt is.

For example, silver chloride has a Ksp on the order of 10^-10, while lead chloride has a Ksp on the order of 10^-5, so silver chloride is a lot less soluble. So if you add a TINY amount of chloride (I should really calculate the amount based on the Ksp) you can expect the silver to precipitate out first. You can then filter it out and then you have dissolved lead left over in your solution---that is a separation technique called fractional precipitation. But again, I don't know if that is appropriate for your class- it would be for the second semester of gen chem or for but not for an introductory class.

Let me know if that helps,
Holly