The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. The mass of an electron of sodium will be:

The mass of an electron of sodium will be 9.1 $\times$ 10^-29 kg

An ion and an atom of sodium differ by one electron. 
The difference in the mass of 100 moles of sodium atoms and sodium ions. would be a difference of 100 moles of electrons.
 

Mass of 100 moles of electrons $=5.48002$g
Mass of 1 mole of electron $=\frac{5.48002}{100}$  g
Mass of one electron $=\frac{5.48002}{100\times 6.022\times {10}^{23}}$
                                  $\Rightarrow 9.1\times {10}^{-29}$ kg

Therefore, the mass of an electron of sodium is 9.1 $\times$ 10^-29 kg.