Which element is found in both potassium chlorate and zinc nitrate?

The common element is: Oxygen    

Explanation:

Potassium chlorate is an ionic molecule, having molecular formula KClO₃. It contains a potassium cation (K⁺) and a chlorate anion (ClO₃⁻). The chlorate anion is composed of a chlorine atom and three oxygen atoms.

Zinc nitrate is an ionic molecule, having molecular formula Zn(NO₃)₂. It contains a zinc cation (Zn²⁺) and a nitrate anion (NO₃⁻). The nitrate anion is composed of a nitrogen atom and three oxygen atoms.

Therefore, the common chemical element that can be found in both, potassium chlorate and zinc nitrate is oxygen.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)