Compute the difference in masses

$\mathrm{Mg}$ atom and $\mathrm{Mg}^{2+}$ ion differ by electrons $=2$

$10^{3}$ moles of $\mathrm{Mg}$ atoms and $\mathrm{Mg}^{2+}$ ions differ in electrons $=10^{3} \times 2$ moles

Mass of one electron $=9.1 \times 10^{-31} \mathrm{~kg}$

Mass of $10^{3} \times 2$ moles of electrons $=10^{3} \times 2 \times 6.022 \times 10^{23} \times 9.1 \times 10^{-31} \mathrm{~kg}$

$=109.6 \times 10^{-5} \mathrm{~kg}=1.096 \times 10^{-3} \mathrm{~kg} .$