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.

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⁺ and CO₃ ^2-

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