Question

Which is the set of nonstoichiometric compounds?

• A. $NaCl,FeO,MgC{l}_2$
• B. $FeO,CuS,VSe$
• C. $F{e}_3{O}_4,NaCl,Cus$
• D. $CuCl,CuS,MgO$

Answer 1

The correct option is B $FeO,CuS,VSe$

A unique feature of the transition elements is that they form non-stoichiometric compounds. These are compounds of indefinite structure and proportions. For example, iron(II) oxide(FeO) should be written with a bar over the formula, FeO to indicate that the ratio of Fe and O atoms is not exactly 1 : 1. An analysis shows that the formula varies between $F{e}_{0.94}O$ and $F{e}_{0.84}O$. Non-stoichiometry of FeO is caused by defects in the solid structure.

Non-stoichiometry is shown particularly among transition metal compounds of the group16 elements (O, S Se, Te). It is mostly due to the variable valency of transition elements.

For example, copper sulphide is precipitated from a solution containing $C{u}^{2+}$ by passing in $H_{2}S$. The sulphide is completely insoluble, but this is not used as a gravimetric method for analyzing Cu because the precipitate is a mixture of CuS and $C{u}_{2}S$.

Hence, the correct option is $\text{B}$