Question

In the process of rubbing the lenses of the eyeglasses, 6.28x1010 electrons were transferred. What is the change in their masses?

Answer 1

The change in mass equals the total mass of all the electrons:

$$\Delta m=m_eN,\\ \Delta m=9.109·10^{-31}·6.28·10^{10}=5.72·10^{-20}\text{ kg}.$$