The bolt of lightning transfers a charge equal to 25 C to the Earth. An electric charge is expressed in Coulombs (C). It is negatively charged particles or electrons that move when a current travels though this is equivalent to a positive charge moving in the opposite direction. Each electron has a charge of -1.609*10^-19 C. The negative sign indicates that electrons are negatively charged. The mass of an electron is 9.109*10^-31 kg.
To determine the mass of electrons that are transfered to the Earth use the electric charge and mass of an electron. 25 C is equivalent to 25 C is `25/(1.602*10^-19) = 1.56*10^20` electrons. The mass of these electrons is equal to `1.56*10^20*9.109*10^-31 = 1.42*10^-10` kg.