A metal 'M' having atomic mass 62. 5 crystallizes in cubic close packing and it shows " Schottky defects". If the edge length of the cubic lattice is 500 pm and density of the metal is 3.09 g/ml then calculate number of moles of metal atom 'M' missing per litre of the crystal. [Given:  $1amu=1.67\times {10}^{-24}gm$]