Explain stoichiometric defects shown by non-ionic solids.

If $10^{-4} \% SrCl_2$ is added to a crystal of $NaCl$,find the number of cationic vacancies created in the crystal.

$A$ sample of ferrous oxide has actual formula $Fe_{0.93}O_{1.00}$. In this sample,what fraction of metal ions are $Fe^{2+}$ ions? What type of non-stoichiometric defect is present in this sample?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ In a particular point defect,an ionic solid is electrically neutral,even if few of its cations are missing from its unit cells.
Reason $(R):$ In an ionic solid,Frenkel defect arises due to dislocation of cation from its lattice site to interstitial site,maintaining overall electrical neutrality.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which defect causes a decrease in the density of a crystal?

If $AgCl$ is doped with $1 \times 10^{-4}$ mole percent of $CdCl_2$,the number of cation vacancies (in $mol^{-1}$) is