Since each Cu atom has 1 free electron

Therefore one mole Cu (63.5gram) has one mole free electrons (6.02×10^23 electrons)

Therefore one gram Cu has 6.02×10^23 ÷63.5 electrons)

Therefore 1mg Cu has 6.02×10^23 ÷(63.5×10^3)electrons

Charge possessed by these electrons is equal to number of these electrons multiplied by electronic charge

=6.02×10^23×1.6÷(63.5×10^3×10^19)Coulomb