A sample of ferrous oxide has actual formula $\mathrm{Fe}_{0.93} \mathrm{O}_{1.00}$. In this sample, what fraction of metal ions are $\mathrm{Fe}^{2+}$ ions? What type of non-stoichiometric defect is present in this sample?
Let the formula of the sample be $\left(\mathrm{Fe}^{2+}\right)_x\left(\mathrm{Fe}^{3+}\right)_y \mathrm{O}$
On looking at the given formula of the compound
$$x+y=0.93\quad\text{.... (i)}$$
Total positive charge on ferrous and ferric ions should balance the two units of negative charge on oxygen
Therefore,
$$\begin{array}{r} 2 x+3 y=2 \quad\text{.... (ii)}\\ \Rightarrow\quad x+\frac{3}{2} y=1\quad\text{.... (iii)} \end{array}$$
On subtracting equation (i) from equation (iii) we have
$$\begin{array}{rlrl} & \frac{3}{2} y-y =1-0.93 \\ \Rightarrow & \frac{1}{2} y =0.07 \\ \Rightarrow & y =0.14 \end{array}$$
On putting the value of y in equation (i), we get
$$\begin{aligned} & x+0.14=0.93 \\ & \Rightarrow \quad x=0.93-0.14 \\ & \Rightarrow \quad x=0.79 \\ & \text { Fraction of } \mathrm{Fe}^{2+} \text { ions present in the sample }=\frac{0.79}{0.93}=0.849 \end{aligned}$$
Metal deficiency defect is present in the sample because iron is less in amount than that required for stoichiometric composition.