Why does table salt,$NaCl$,sometimes appear yellow in colour?

When $NaCl$ (molten) is doped with $10^{-5} \, mol \, \%$ of $AlCl_3$,what is the concentration of cation vacancies?

Analysis shows that nickel oxide has the formula $Ni_{0.98}O_{1.00}$. The fractions of $Ni^{2+}$ and $Ni^{3+}$ ions in the crystal are ......

Schottky defect in a crystal is observed when,

Which of the following statements are not correct?
$(A)$ Diode is a combination of $n$-type and $p$-type semiconductors.
$(B)$ Silicon or germanium is electron rich impurity.
$(C)$ Phosphorus and arsenic are electron deficient impurities.
$(D)$ Schottky defect decreases the density of the crystal.

$NaCl$ is doped with $2 \times 10^{-3} \, mol \% \, SrCl_2$. What is the concentration of cation vacancies?