Compounds/Ions Question?

Question

Give the formulas of all the compounds containing no ions other than K+, Ca2+, Cl-, and S2-

I dont know how to figure out the answer to this..Thanks ahead of time to anyone who tries to help!

Answer 1

Anna, it's really pretty simple. You know a compound has to be electrically neutral, so you've got to use a positive ion and a negative ion to make a compound that comes out to have a neutral charge. Well, take the first positive ion: K+. You can only make two compounds with it and the two negative ions you're given, that is with Cl- and S=. Since K is +1, you only need one Cl- to make a compound: KCl. the second negative is -2 so you need 2 K+ ions to balance out the 2-, which gives K2S, (sorry, I can't write the "2" as a subscript). The second positive ion, Ca is +2, so the situation is reversed, that is you need 2 Cl- ions to balance the +2: CaCl2. And the second compound comes out equal if you use one of each since they both have a charge of 2, so you get CaS.