You must remember what you learned in Modules 4 and 5. In Module 4 (pp. 136-145), you learned how to determine the chemical formula of simple ionic compounds. In Module 5 (pp. 177-181), you learned the same technique and applied it to ionic compounds which involve polyatomic ions. At the same time, you were required to memorize the polyatomic ions on p. 175.
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Since you know K is a metal, any compound with K, C, and O is ionic. Thus, it follows these laws. As you learned in Module 4, in ionic compounds K has a charge of 1+. As you learned in Module 5, the only stable form of CO3 is the carbonate ion, CO3 2-. You don’t actually have to remember that, however, it just makes things easier. You are supposed to know that H2CO3 gives up two H 1+ ions. If you take 2 H’s and two positive charges away from H2CO3, you get CO3 2-. Thus, either way, you know you are dealing with
K1+Â and CO3Â 2-
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According to what was learned in Module 5, you can get the chemical formula by dropping the signs and switching the charges. Thus, it is
K2CO3