Answer
Number of electrons needed = $6.25\times10^{18}$
Total mass = $5.6875\times10^{-12}$ kg
Work Step by Step
Charge on 1 electron = $-1.6\times10^{-19}$ C
Required charge = $-1$ C
Number of electrons needed = $\frac{-1}{-1.6\times10^{-19}}$ = $6.25\times10^{18}$
Mass of $1$ electron = $9.1\times10^{-31}$ kg
Thus, total mass = $6.25\times10^{18} \times 9.1\times10^{-31} $ = $5.6875\times10^{-12}$ kg
