Answer: 6.02 x 1018 mol-1
The Solution given is as follows:
On doping NaCl by SrCl2, one Sr2+ ion replaces two Na+ ion.
So, no. of moles of cation vacancy in 100ml NaCl = 10-3
no of moles of cation vacancy in 1 mole NaCl = 10-3/100 = 10-5
Thus, total cation vacancy
= 10-5 x NA
= 10-5x 6.022 x 1023
= 6.022 x 1018