Calcium reacts with a polyatomic ion to form a compound with the general formula Ca(X). What would be the most likely formula for the compound formed between potassium and the polyatomic ion X?

A) K X
B) K2X
C) KX2
D) K2X3
E) K3X2

B) K2X

Explanation:

In an uncharged compound, the total oxidation state must be zero. The oxidation state of the calcium is +2, thus we get the following formula, where x is the oxidation state of the polyatomic ion X:

Also, it is known that potassium has an oxidation state of +1. Since the new compound also has a total oxidation state equal to zero, we get the following equation, where k is the number of K atoms:

That's how it is found that the compumd consists of 2 K+ ions and one X ion.