[Omnihuron]

I am doing reaction equations in Chemistry ad in the book it says "Identify the precipitate that forms when BaCl₂ and K₂SO₄ are mixed. But I have no idea what the ions that forms these molecules are. Later is says matter-of-factly that the reactants contain Ba²⁺, Cl^-, K⁺ and SO₄^2-. How did it get those numbers? It didn't explain how they figured it out. Is it memorization or is there a method? Thanks a bunch!

[MathisFun]

Hello Omnihuron, welcome to the forums. I'm not a resident expert here, as I have just begun the long, but enjoyable process of learning chemistry. Given what I have learned thus far, I can offer a little commentary on this subject.

If you look at a Periodic Table of Elements, there are divisions, columns and rows. In this instance, we will concern ourselves with the columns, which are referred to as 'groups.'

Group 1-A is the elements from the first element, Hydrogen, going straight downwards to Francium. These elements tend to give off an electron, thus H⁺, Li⁺, Na⁺, K⁺, etc.

The next column is Group 2-A, which follows the pattern, giving off 2 electrons, Ca²⁺, Sr²⁺, Ba²⁺, etc.

Group 3-A skips the Transitional elements, which are groups 3B-8B, then 1B and 2B, and starts from Boron, thus B³⁺, Al³⁺, etc.

From Group 7-A, beginning with Fluorine, the elements accept an electron, thus F^-, Cl^-, Br^-, etc.

Group 6-A, as you can guess, is O^2-, S^2-, etc.

As for SO₄^2- (Sulfate), which is a polyatomic ion, the only advice I have been given is to memorize the common polyatomic ions, which happens through repeated exposure of said ion.

Your question is asking, if ions in aqueous solution will react with each other?

I'll give you an example, which is closely related to your equation.

Identify the precipitate that forms, when a solution of barium chloride, BaCl₂, is added to a solution of sodium sulfate, Na₂SO₄.

Because both barium chloride and sodium sulfate are ionic compounds, they will dissociate into ions, when both are placed into a solution of water.

Ba²⁺ (aq) + 2Cl^- (aq) + 2Na⁺ (aq) + SO₄^2- (aq)  BaSO₄ (s) + 2Na⁺ (aq) + 2Cl^- (aq)

(aq) stands for aqueous solution
(s) solid

As you can see, from the product side of the reaction, the solid, BaSO₄ (Barium Sulfate) forms.

There are solubility rules for compounds, and in this case, Na⁺ and Cl^- are soluble in water, meaning it doesn't form a solid, while most SO₄^2- (Sulfates) are soluble, with the exception of CaSO₄, SrSO₄, BaSO₄, and PbSO₄, thus they form solids.

That should get you started. Hopefully, someone more informed will chime in and correct me.

[Omnihuron]

Hey thanks a lot! So there are rules by group and some that I just have to memorize; okay I will look up and memorize the polyatomic ions. Thanks a bunch!

[Borek]

thus B³⁺

No such thing as B³⁺ cation. Boron is classified as metalloid - neither metal nor non metal.

[Arkcon]

As for SO₄^2- (Sulfate), which is a polyatomic ion, the only advice I have been given is to memorize the common polyatomic ions, which happens through repeated exposure of said ion.

That's not the only way, although it is expected.  The O.P. was given the compound names -- BaCl2 and K2SO4.  If you knew barium was Ba²⁺, and potassium was 1⁺, you can see how the sulfate ion has to be ²⁺.  And pretty soon, you start to remember the common ones.

[MathisFun]

thus B³⁺

No such thing as B³⁺ cation. Boron is classified as metalloid - neither metal nor non metal.

Thanks for the correction, Borek.