What is the percentage of $Fe^{3+}$ in $Fe_{0.95}O$?

If $10^{-4} \, \text{mole} \%$ of $SrCl_2$ is added to $NaCl$,the concentration of cation vacancies created is...... $(N_A = 6.02 \times 10^{23} \, \text{mol}^{-1})$

Use the appropriate symbols $T$ (True) or $F$ (False) for the following statements regarding defects in solids:
$(1)$ Frenkel defect arises due to the arrangement of cations and anions with a very small difference in size.
$(2)$ Frenkel defect can be called a dislocation defect.
$(3)$ $F$-centers arise due to the migration of electrons released from metal atoms into the crystal lattice.
$(4)$ Physical properties of the crystal do not change due to Schottky defect.

Which of the following statements is $NOT$ true about Schottky defect?

Analysis shows that nickel oxide has the formula $Ni_{0.98}O_{1.00}$. What fractions of nickel exist as $Ni^{2+}$ and $Ni^{3+}$ ions?

If a trivalent atom replaces a few silicon atoms in the three-dimensional network of silicon dioxide,what would be the type of charge on the overall structure?