Compare the concentration of positive ion vacancies in an nacl crystal due to the presence of 10-4 mol fraction of cacl2 impurity with the intrinsic concentration present in equilibrium in a pure nacl crystal at 400oc. the formation energy ? hs of a schottky defect is 2.12 evl, and the mole fraction of schottky defects near the melting point of 800oc is 2.8 x 10-4.