Question:
The difference in the mass of 100 moles each of sodium atoms and sodium ions is $5.48002 \mathrm{~g}$. Compute the mass of an electron.
Solution:
Sodium $(\mathrm{Na})$ and sodium ion $\left(\mathrm{Na}^{+}\right)$differ in one electron. This means that 100 moles of $\mathrm{Na}$ atoms and $\mathrm{Na}^{+}$ ions differ in electrons $=100 \mathrm{~mol}$.
No. of electrons in 100 moles $=100 \times 6.022 \times 10^{23}=6.022 \times 10^{25}$
Mass of $6.022 \times 10^{25}$ electrons $=5.48002 \mathrm{~g}$
Mass of one electron $=\frac{(5.48002 \mathrm{~g})}{\left(6.022 \times 10^{25}\right)}=9.1 \times 10^{-28}=9.1 \times 10^{-31} \mathrm{~kg}$.