Download A copper coin has a mass of 3 g

Problem:
A copper coin has a mass of 3 g. What is the total charge of all the electrons in it? Given
that the atomic number of Cu is Z = 29 and the atomic mass is 63.5 g/mole.
Solution:
The total charge, q, is the number of electrons multiplied by the charge of each
electron
ie, q = ne (e = -1.6 x 10-19)
Thus we have to calculate the total number of electrons in the coin. It is given that Z =29
which means that there are 29 electrons in a copper atom. Now the problem needs the
total number of atoms in a 3g copper coin. (So that we can give it back to find the total
number of electrons)
But we know that the number of atoms is the number of moles times Avagadro’s number.
Now what is the number of moles??
It is simply the ratio of the mass and the atomic mass. In our case it is, m/M
Where m is the mass of the coin and M is the atomic mass. (Yes 3/29). We can substitute
these things back in the first equation,
q = [m NA Z e] / M
Now we can substitute the values, NA = 6.02 x 1023 atoms/mole, m = 3g, Z = 29
electrons/atom, M = 36.5 g/mol.
You do the simplification step and see whether it is giving q as –1.32x105 C (A very
high value indeed) But there is an equal amount of positive charge to hold this much
amount of negative charge, and thus we are not aware of it.