Q:Find the number of vacancy When NaCl is doped with 10⁻⁵ mole % of SrCl₂ ?

Q:Find the number of vacancy When NaCl is doped by 10⁻⁵ mole % of SrCl₂ ?

Ans:

In 100 moles of NaCl ion 10⁻⁵ moles of SrCl₂ is added

=> In 1 mole NaCl 10⁻⁷ Moles of SrCl₂ are added.

Now, by adding one molecule of SrCl₂ one Na⁺ ion is replaced by one Sr⁺² ion. 

To maintain the electrical neutrality one Na⁺ ion is removed leaving behind a vacancy.

1 molecule creates one vaccancy.

So,  10⁻⁷ moles of molecules will create 10⁻⁷ moles of vacancy.

Therefore , total number of vacancies are 10⁻⁷ x Nₐ or 6.022 x 10 ¹⁶.

Trick to find number of vacancies 
No. vacancies = difference between  the magnitude of charge of the added ion and the removed ion.