[futurebrewer]

Hey guys, I have a lab that is due tomorrow and I am really stuck on how to move forward with this. Here is the problem:

Use your knowledge of chemistry to identify an unknown that consisted of one of the following alkaline earth compounds: MgCl2, Ba(NO3)2, CaCO3, or SrBr2.

A solid sample readily dissolved in water. The dissolved sample was divided into two test tubes.  Sulfuric acid was added to one test tube and there was no observable reaction.  Silver nitrate solution was added to the other test tube and a precipitate formed.

What was the compound? How did you come to the conclusion that it was the unknown compound?

I thought when you do a net ionic equation, you take the first half of one compound and the second half of the other compound. But there is no Ag in the compounds listed.

I know that Nitrates are soluble. I also know that chlorides are soluble except those with Ag+. It seems like I'm onto to something there, but I can't really figure this out. Any help would be appreciated. Thanks!

[Arkcon]

OK, you know two things: Nitrates are soluble.  Chlorides are soluble, except for silver (and two others, try to find them).  But your possible unknown produces an insoluble compound, and uses sulfuric acid.  You'll have to flesh out your table of possibilities to solve this problem.  A better table may be available in your textbook.

[futurebrewer]

Ok, Chlorides are soluble except for Ag+, Pb2+, Cu+, and Hg2^2+.

Do I need to worry about the sulfuric acid? In the problem it says that there is no reaction when the Sulfuric acid is added to one of the test tubes. However, when the AgNO3 is added to the other test tube, a precipitate forms.

[futurebrewer]

Anyone?

[UG]

Do I need to worry about the sulfuric acid?

Yes very much so because the addition of sulfuric acid which produced no observable reaction basically rules out 3 of the possible 4 compounds. Check the solubility rules for the sulfate compounds, consider also the acidic nature of sulfuric acid and its reaction with base.

[futurebrewer]

ok, well it the unknown substance cannot be MgCl2 or Ba(NO3)2 because they are both soluble.

That leaves us with CaCO3 and SrBr2.

ok, looks like SrBr2 is soluble.

That leaves. CaCO3.

This is the mystery substance. It is because All common carbonates are insoluble except those of group 1A.

Is this correct? Did I get the correct answer?

[Borek]

ok, well it the unknown substance cannot be MgCl2 or Ba(NO3)2 because they are both soluble.

You were told your sample dissolves easily, so why do you eliminate soluble substances? Sounds to me like you should eliminate insoluble substances.

[UG]

ok, well it the unknown substance cannot be MgCl2 or Ba(NO3)2 because they are both soluble.

Hmmm...
I don't mean check the solubility of these compounds, I mean check the solubility of their cations + sulfate. What happens when you add sulfuric acid is that there are now sulfate ions in solution, and some of the unknown cations will react with the sulfate ion to produce a precipitate because they are insoluble. But because you were told there was no observable reaction, you can rule out these insoluble sulfates because no precipitates formed.

ok, looks like SrBr2 is soluble.

Check the solubility of the sulfate, same as the others.