A certain hypothetical atom contains 78 protons and 113 neutrons in its nucleus and has an atomic mass of ( 191 - .068 ) atomic mass units, or amu (an amu is approximately 1.66 * 10^-27 kg; a proton mass is about 1 + .00073 amu, neutron mass about 1 + .00087 amu). What is the mass defect of this atom? What is the energy equivalent of the mass defect of a mole of these atoms?

The total mass of 78 protons and 113 neutrons is 78 * (1 + .00073) amu + 113 * ( 1 + .00087) amu = ( 191 + .15525 ) amu.

This exceeds the actual mass of the atom by ( 191 + .15525 ) amu - ( 191 - .068 ) amu = .22325 amu.

A mole of these atoms would contain an Avagadro's Number of nuclei with total mass defect

• mass defect per mole = mass defect per atom * Avagadro's Number = .22325 amu * (6.02 * 10^23) = ( .22325 * (1.66 * 10^-27 kg) ) * (6.02 * 10^23) = 2.230982 * 10^-4 kg.

The energy in this mass would be

• energy per mole = m c^2 = 2.230982 * 10^-4 kg ( 3 * 10^8 m/s ) ^ 2 = 2.007884 * 10^13 Joules.