The anions (A) of C _ p A _ q molecule forms an fcc lattice. Cations (C) are positioned at the body center and half of the edge centers. The formula of the molecule is

Step-by-step solution

In a unit cell of fcc lattice, A is present at 8 corners and 6 faces. Effective no. of ^ A ^ = 1 8 8+ 1 2 6=4 C is present at body centre and 6 edges. Effective no. of ' C ^ =1+6 1 4 = 5 2 C _ 5 2 ~A ₄= C ₅ ~A ₈