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Q.
Formula of nickel oxide with metal deficiency defect in its crystal is $A_{0.8}O$. The crystal contains $A^{2+}$ and $A^{3+}$ ions. The fraction of nickel existing as $A^{2+}$ ions in the crystal is
So the ratio of $A : O =80: 100$
Thus, if there are 100 atoms of oxygen then 80 atoms of $A$.
Let $x$ be the no. of atoms of $A ^{+2}$.
No. of atoms of $A ^{+3}=80- x$
Charge on A = Charge on O
Since oxygen has charge $-2$.
Therefore,
$
\begin{array}{l}
2 x+3(80-x)=2 \times 100 \\
2 x+240-3 x=200 \\
\Rightarrow x=40
\end{array}
$
Therefore,
Fraction of $A ^{+2}=\frac{\text { Atoms of } A ^{+2}}{\text { Total no. of atoms }}$
$\Rightarrow $ Fraction of $A ^{+2}=\frac{40}{80}=0.50$