Question

A copper penny has a mass of 2.6 g Assuming it is pure copper, how much of this mass is contributed by a) the neutrons in the copper nuclei b) the electrons

Answer 1

First calculate the moles of Cu
2.6 g / 63.058 g/mole = 0.04123 moles of Cu

Multiplying it with Avogadros number you get the number of atoms
0.04123 mol x 6.040 x 10^23 copper atoms/mole of copper= 2.490292 * 10^22 atoms

An electron has a mass of 9.085*10^-28 gram 
Electrons in the atoms are 29 x 2.490292 * 10^22
Their mass would be (9.085*10^-28)( 29 x 2.490292 * 10^22) = 6.561 * 10^-4 grams

Copper has a normal atomic mass of 63.546 It's atomic number is 29. It's atomic mass is 63 disregarding other mass in the atom
Atomic mass - Atomic number = neutrons mass
29 - 63 = 34 (Neutron weight)

Neutron weight is 34 g/mol
Multiply with the number of moles you get the actual mass of neutrons in the coin
34 x 0.04123 = 1.4 g = mass of neutron in the coin

Answer 2

The copper has a normal atomic mass of 63.546

it's atomic number is 29

it's atomic mass is 63 disregarding other mass in atom
Atomic mass - atomic number = neutron mass

63 - 29  = 34

Hope this helps