Let's calculate the total charge of M=4.8 g=0.0048 kg of protons.
Each proton has a charge of [tex]q=1.6 \cdot 10^{-19} C[/tex], and a mass of [tex]m_p = 1.67 \cdot 10^{-27}kg[/tex]. So, the number of protons is
[tex]N_p = \frac{M}{m_p}= \frac{0.0048 kg}{1.67 \cdot 10^{-27}kg}=2.87 \cdot 10^{24}[/tex]
And so the total charge of these protons is
[tex]Q_p = qN_p = (1.6 \cdot 10^{-19}C)(2.87 \cdot 10^{24})=4.6\cdot 10^5 C[/tex]
So, the neutralize this charge, we must have [tex]N_e[/tex] electrons such that their total charge is
[tex]Q_e = -4.6 \cdot 10^5 C[/tex]
Since the charge of each electron is [tex]q_e = -1.6 \cdot 10^{-19}C[/tex], the number of electrons needed is
[tex]N_e = \frac{Q_e}{q}= \frac{-4.6 \cdot 10^5 C}{-1.6 \cdot 10^{-19}C}=2.87 \cdot 10^{24} [/tex]
which is the same as the number of protons (because proton and electron have same charge magnitude). Since the mass of a single electron is [tex]m_e=9.1 \cdot 10^{-31}kg[/tex], the total mass of electrons should be
[tex]M_e = N_e m_e = (2.87 \cdot 10^{24})(9.1 \cdot 10^{-31}kg)=2.6 \cdot 10^{-6}kg[/tex]
