NEET Chemistry The Solid State Questions Solved

A spinel is an important class of oxides consisting of two types of metal ions with the oxide ions arranged in ccp layers. The normal spinel has 1/8th of the tetrahedral void occupied  by one type of metal and one half of the octahedral voids occupied by another type of metal ions. Such a spinel is formed by Zn2+, Al3+ and O2- with Zn2+ in tetrahedral void. Give the simplest formula of the spinel.

(A) ZnAl2O4

(B) ZnAl2O3

(C) ZnAlO

(D) None of these

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(A) Let  the no. of tetrahedral voids = a

No. of octahedral voids = a/2

1/8 th tetrahedral voids is occupied by Zn2+

No. of Zn2+ =a/8

1/2 of octahedral voids occupied by Al3+

No. of Al3+ = a/4

Ratio of Zn2+: Al3+

             1     :   2

For 1 mole Zn2+, Al3+=2

Total +ve charge = 2+3x2=8  therefore, No. of O2- =4

The simplest formula will be  ZnAl2O4

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