A 2.50 g copper coin is given a charge of -2.00 ́ 10-9 C.?

(a)
Each electron carries -1.602 × 10⁻¹⁹ C of charge.

Number of excess electrons on the coin
= (-2.00 × 10⁻⁹ C) / (-1.602 × 10⁻¹⁹ C/electron)
= 1.25 × 10¹⁰ electrons


b)
Mass of an electron = 9.109 × 10⁻²⁸ g

Mass of the excess electrons
= (1.25 × 10¹⁰ electrons) × (9.109 × 10⁻²⁸ g/electron)
= 1.14 × 10⁻¹⁷ g

Percent by mass of the excess electrons in the coin
= [(1.14 × 10⁻¹⁷)/2.50] × 100(%)
= (4.56 × 10⁻¹⁶)(%)