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If the mass of proton=1.008a.m.u. and mass of neutron=1.009a.m.u., then binding energy per nucleon for 4Be9 (mass=9.012amu) would be:
A. 40.065Mev
B. 60.44Mev
C. 67.2Mev
D. 6.72Mev

Answer
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Hint: First we have to find a mass defect in a.m.u and binding energy of 4Be9 in Mev and next we can calculate binding energy per nucleon in the unit of Mev. We have to know the rest mass of the stable nucleus of a stable atom is always less than the sum of the masses of constituent nucleons, so this difference is called the mass defect which is denoted by Δm.

Complete answer:
Binding energy is nothing but is the energy which is used in keeping the nucleons bound together. If the mass of a nucleus is always less than the mass of constituent (protons and neutrons) nucleons. This mass difference is called a mass defect and is denoted as Δm.
The general formula of finding mass defect is.
Δm = Z. mp + (A – Z). mn – M (Z, A)
Where mn is the mass of a neutron.
Mp is the mass of a proton.
M (Z, A) is the mass of bounded nucleus
Z is the atomic number
A is the atomic mass number.
According to question
Δm=4mp+(94)mnmBeΔm=(4×1.008+5×1.0099.012)amu0.065amu
Calculate Binding energy of 4Be9
=Δm×931.5Mev
Δm is the mass defect. Put this value in above equation
0.065×931.5Mev60.5475Mev
Now, we get the value of Binding energy of 4Be9
But, binding energy per nucleon will be calculate as BEnucleon
Put these value in above equation we get
60.54759∴=6.7275Mev

Hence the given option D is the correct answer.

Note:
There is a special case to know Einstein's law of inter-conversion of mass into energy where mass-defect is in the form of energy and is responsible for binding the protons and neutrons together to form a nucleus. We can study with the help of binding energy per nucleon curve with mass number.