A copper sphere of mass 2.0 g contains about 2 x ${10}^{22}$ atoms. The charge on the nucleus of each atom is 29e (e = electronic charge). The mass of an electron is 9.11 x ${10}^{-31}$ kg. How much mass will the sphere lose or gain if it is a given a charge of +2 μC?

To give positive charge to the copper sphere, a certain number of free electrons must be removed.
The total number of electrons is 29(2 x ${10}^{22}$) = 5.8 X ${10}^{23}$
Since a positive charge of 2 μC is to be given, the number of electrons to be removed $\frac{2\times {10}^{-6}}{1.6\times {10}^{-19}}=1.25\times {10}^{13}$
The mass of the electrons removed = 1.25 X ${10}^{13}$ X 0.91 X ${10}^{-30}$ kg

= 1.13 X ${10}^{-17}$ kg = 1.13 X ${10}^{-14}$ gm
This mass will be lost by the copper sphere.