Q54

Q54 x x z. 0

Dear Student,

NaCl is doped with $2 \times {10}^{-3} mole \%$ 
It means 100 moles are doped with this much amount of moles,
That means one mole NaCl is doped with 0.00002 moles of Sr⁺².
One Sr⁺² create one cation vacancy , hence 
cation vacancies per mole will be equal to $0.00002\times 6.022\times {10}^{23}$ vacancies or $12.046\times {10}^{18}$ 
Hope this helps,

Regards