Suppose we want to create a positive charge of 150 microC on

Suppose we want to create a positive charge of 15.0 microC on a block of Copper metal with mass 3.50 kg. What fraction of the electrons in the copper block would we remove? (Z=29, A=64, N = 35 for copper).

Solution

e = charge on electron = 1.6*10^-19 C

no. of electron removed = n = q/e = 15*10^-6/(1.6*10^-19) = 9.375*10^13 electrons

mass = 3.5 kg

molar mass of copper = 63.546*10^-3 kg/mol

no. of moles = 3.5/(63.546*10^-3) = 55.08 moles

1 mol of a substance contains Avogadros number of particles = 6.023*10^23

55.08 moles contains = 6.023*10^23*55.08 = 3.32*10^25 atoms

A neutral copper has 29 electrons

no. of electrons in the ball = 29*3.32*10^25 = 9.63*10^26 electrons

fraction of electrons removed = 9.375*10^3/(9.63*10^26) = 9.735*10^-14

Let me know if you have any doubt.