If NaCl is doped with 10^ -4 ~mol \% of SrCl_2 , the concentration...

Since each $\mathrm{Sr}^{++}$ ion provides one cation vacancy, hence Concentration of cation vacancies = mole \% of $\mathrm{SrCl}_2$ added $$\begin{aligned} & =10^{-4} \text{ mole } \% \\ & =\frac{10^{-4}}{100} \times 6.023 \times 10^{23} \\ & =6.023 \times 10^{17} \mathrm{ mol}^{-1} \end{aligned}$$ Related Theory Doping is the process of adding impurities to intrinsic semiconductors to alter their properties. When an intrinsic semiconductor is doped with Trivalent impurity it becomes a P-Type semiconductor. The $P$ stands for Positive, which means the semiconductor is rich in holes or Positive charged ions.