A compound contains elements X an Y. Y atoms form CCP lattice and atoms of X occupy 1/3rd of tetrahedral voids. What is the molecular formula of the compound ?

Number of effective Y atoms = (8 corners X 1/8 per corner atom share +6 faces X 12 per middle of the atom share)                                                =( 8 X 1/8 +6 X 12)                                                = 4 One Y atoms create 2 Y tetrahedral voids = 2 X 4=8Number of effective X atoms =13x 8 =83XFormula of unit cell = X83Y4 =X2Y3