First, the charge of the electron e = -1.602 x 10^-19
Net Charge Q = -70.1 uC
Mass of the electron m = 9.1 x 10^-31 kg
Number of Electrons is net charge divided by electron charge
N = -70.1 / -1.602 x 10^-19 => N = 70.1 x 10^-6 / -1.602 x 10^-19
N = 4.375 x 10^14
So calculating the increase in mass = N x mass of the electron, m => N x m
= 4.375 x 10^14 x 9.1 x 10^-31
= 39.8 x 10^-17