answer

On doping NaCl by SrCl2, one Sr2+ ion replaces two Na+ ions.
So, number of moles of cation vacancy in 100 ml NaCl = 10-4
Number of moles of cation vacancy in 1 mole NaCl = 10-4/100 = 10-6
Thus, total cation vacancy = 10-6 x NA = 10-6 x 6.022 x 1023 = 6.022 x 1017 mol-1