Question

A 50.6 g ball of copper has a net charge of 1.6 µc. what fraction of the copper's electrons have been removed? (each copper atom has 29 protons, and copper has an atomic mass of 63.5.

Answer 1

First figure out how many atoms you have with Avogadro's number.  Since there are 63.5 grams/mol and you have 50.6 grams, you have (50.6/63.5)6.022E23=4.7986E23 atoms.  Since there are 29 protons per atom, there are also 29 electrons per atom, so you should have a total of
29*4.7986E23=1.3916E25 electrons.
Since there is a positive charge you know some of these electrons are missing.  How many are missing can be found by dividing the charge you have by the charge on the electron:  1.6E-6/1.6022E-19 = 9.98627E12 electrons are missing.  
Now take the ratio of what is missing to what there should be:
9.98627E12/1.3916E25 = 7.1760873E-13