Question:
Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is $9.1 \times 10^{-28} \mathrm{~g}$ ). Which one is heavier?
Solution:
$\mathrm{Al}$ atom and $\mathrm{Al}^{3+}$ ion differ by electrons $=3$
1 mole of $\mathrm{Al}$ atoms and 1 mole of $\mathrm{Al}^{3+}$ ions differ (by) electrons $=3 \times \mathrm{N}_{\mathrm{A}}$
Mass of one electrons $=9.1 \times 10^{-28} \mathrm{~g}$ (given)
Mass of $3 \times \mathrm{N}_{\mathrm{A}}$ number of electrons $=3 \times 6.022 \times 10^{23} \times 9.1 \times 10^{-28} \mathrm{~g}$
$=164.4 \times 10^{-5} \mathrm{~g}=0.001644 \mathrm{~g}$