Calculating vacancies in crystal structure I have this question on a practice exam. I'm not quite sure how to find the number of vacancies when replacing atoms. 

Question:
The stable oxide for lithium is $\ce{Li2O}$. How many oxygen vacancies in $\ce{SiO2}$ will be created if 100 Si atoms are replaced by 100 Li atoms (remember, O has a -2 charge in an ionic material)?

I think that because Li is +1 charge and Si is +4 that when I replace the Si with Li it leaves 150 unbound O. So 150 vacancies? Am I totally wrong?
That's it, you have the right answer. Conservation of charge (electroneutrality) is what you use to determine the number of vacancies.