Seeking Clarity on Calculating Molar Binding Energy of Carbon-12

[Moe_slow]

Hey, i was ill for a few day, (damn flu) and missed some classes, am trying to catch up and need some help to understand some things.

Question:
Calculate the molar binding energy of carbon-12 nuclei.

Attempt at solution:
I don't really know where to start with this. can someone explain some terms or background information for me? thanks.

 

[Kurdt]

The binding energy of a nucleus is the enrgy that holds the nucleons (neutrons and protons) together in the nucleus. This energy comes from the mass difference between the individual nucleons and the nucleus itself. The mass difference is related to the energy through Einstein's famous E=mc^2. The molar binding energy is the binding energy of avagadros number of C-12 nuclei.

 

[Moe_slow]

ok thanks. i think i understand.

so the mass of one proton is 1.0078 and neutron is 1.0086. there are six neutrons and 6 protons. that gives us 12.098940 and since the mass of the carbon 12 is 12.0000 the mass difference is 0.098940. insert that into einstien's equation and we get ((9.8x10^-5)(9.0x10^13))=8.9x10^9.moving on. can you help me understand the concept of this question.

calculate the nuclear binding energy per nucleon. 4He 4.0026u.
i don't really understand the j/nucleon part or how to get there. i tried doing it similar to the other but get a wrong answer.

 

[Gokul43201]

ok thanks. i think i understand.

so the mass of one proton is 1.0078 and neutron is 1.0086. there are six neutrons and 6 protons. that gives us 12.098940 and since the mass of the carbon 12 is 12.0000 the mass difference is 0.098940. insert that into einstien's equation and we get ((9.8x10^-5)(9.0x10^13))=8.9x10^9.

Be careful with your units. Pay attention to the fact that 1.0078g is the mass of a mole of nuclei. You want the mass in kg, for SI units (J).

calculate the nuclear binding energy per nucleon. 4He 4.0026u.
i don't really understand the j/nucleon part or how to get there. i tried doing it similar to the other but get a wrong answer.

In the above case, you found the energy for 1 mole of nuclei - that's N(avogadro) nuclei. In this problem you want to find the binding energy for a single nucleon (4 nucleons in a nucleus, and N nuclei in a mole).

 

[Astronuc]

As Gokul mentioned, one must be careful with units.

For binding energy of He-4, consider mass of 2p and 2n and compare to mass of He-4. Then use that to calculate the binding energy. Divide by the number of nucleons in the He-4 nucleus (4) to get BE/nucleon.

Useful units. 1 u ( or atomic mass unit, amu) = 931.494 MeV = 1.660539 x 10^-27 kg

1 MeV = 1.6022 x 10^-13 J, or more precisely according to NIST,
1.602 176 53(14) x 10^-13 J

See - http://physics.nist.gov/cuu/Constants/index.html

See also Atomic and nuclear constants or use the values in one's textbook.