Answer
$1.75 \times 10^{14}$ $electrons$
$1.59 \times 10^{-16} kg$
Work Step by Step
Firstly to get the number of electrons, divide 28µC by e, that is, $1.6 \times 10^{-19}C$:
$2.8 \times 10^{-5}C \div (1.6 \times 10^{-19}C) = 1.75 \times 10^{14}$
Then multiply the number of electrons with the mass of one electron, $9.1 \times 10^{-31}$:
$1.75 \times 10^{14} \times9.1 \times 10^{-31}kg = 1.59 \times 10^{-16}kg$
