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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?

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That's it, you have the right answer. Conservation of charge (electroneutrality) is what you use to determine the number of vacancies.

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    $\begingroup$ Well, Jon, I think it's the question's fault, not the answers. What can F'x's answer possibly contain more? $\endgroup$
    – M.A.R.
    Commented Aug 10, 2016 at 9:20
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    $\begingroup$ re more explanation; on the face of it one might think that the crystal has a charge of -300 after replacement, so another sentence might help. $\endgroup$
    – porphyrin
    Commented Nov 8, 2016 at 13:42

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