The mass of half mole of electrons is about

(mass of electron = 9.1 * 10^-28g)

One electron has a mass of 9.10938291 x 10^ - 31 kg. One mole of electrons contains 6.022x10^23 electrons where half of a mole is has 3.011x10^23 electrons. Therefore, the mass of half a mole of electrons would be 2.74x10^-7. Hope this answers the question. Have a nice day.