calculation monocationic elements
Oxygen is always divalent negative. An oxide has to be charge balanced, meaning the total has to be zero.


Therefore, some like CaO means you have Ca2+O2-. 
Something like SiO2 is going to be Si4+2O2-.
Something like Na2O is going to be 2Na+O2-.


As you can see, the charges all sum to zero, given that oxygen is negative two.

Let's work out an example:

You have 1 moles of each: K2O, MgO, and ZrO2. You will have one mole of Mg2+, one mole of Zr4+, and two moles of K+.