If $NaCl$ is doped with $10^{-4} \ mol \%$ of $SrCl_2,$ the concentration of cation vacancies will be $(N_A = 6.02 \times 10^{23} \ mol^{-1}).$

The defect that arises when smaller cations move into interstitial sites in a crystal lattice is known as ..............

Identify the type of defect in brass alloy.

The correct statement among the following,regarding defects in solids is:
$(1)$ Frenkel defect is favored by small difference in the sizes of cation and anion.
$(2)$ Frenkel defect is a metal excess defect.
$(3)$ Trapping of electron in the lattice leads to formation of $F$-centres.
$(4)$ Schottky defect has no effect on the physical property of solids.

What causes the formation of a $Schottky$ defect?

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: