# (a) How many electrons would have to be removed from a penny to leave it with a charge of #1.0×10^-7 C#? (b) To what fraction of the electrons in the penny does this correspond? [A penny has a mass of 3.11g; assume it is made entirely of copper.]

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The charge on an electron is

For part b, we need to know how may electrons are in the penny. The relative atomic mass of copper is 63.546. By definition, 63.546g of copper would contain a number of atoms equal to Avogadro's constant which is

So in 3.11g there would be the following number of atoms:

The atomic number of copper is 29, so a neutral copper atom has 29 electrons.

Hence the fraction of electrons that would need to be removed would be:

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