Total number of electrons present in 48g of  ${\mathrm{Mg}}^{2+}$ is: 

Mg atomic number is 12 and molar mass is 24g/mol. 

Therefore, $M{g}^{2+}$ ion has:

10 electrons 48 g Mg²⁺ = 48/24 = 2 mol     

Number of ions in 48g 

Mg²⁺ =2$\times$N_A

number of electrons  $=2\times N_A\times 10=20N_A$