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Can't answer a simple question on Mass Energy!

I can't get the same values for u as they get?

Could you give a bit more information about what you are doing and what answer you are getting so we can see where you are going wrong perhaps.

Reply 2

samArai

Sooo for the mass of the 238.

146X1.00867 + 92x1.00728 = 239.xxx basically its slightly more than the answer I'm suppose to get. I end up with 4.1MEV not 4.3

Reply 3

Hazza616

Don't they give you the values for mass in the question? Then you work out the mas difference in AU and multiply by 931.3

Reply 4

samArai

I posted a screenshot, if you could point to where they do , that would just be terrific.

Reply 5

SDA

I can't see anything wrong with your working, my guess is that the discrepancy arises because of rounding differences in the values, they may well have rounded the numbers slightly more or less because at the moment I don't see how they got their answer, as I agree more with your answer at the moment.

Reply 6

samArai

While I'm flattered. I don't think its due to rounding differences, there's something else. Or I'm really underestimating the both of us. Many that stonebridge guy will come along as help us

Reply 7

SDA

The error may arise from the fact that adding up the mass of all the protons and neutrons in the nucleus does not give the correct value for the mass. This is a result of the binding energy holding the nucleus together affecting the mass. That is why the mass you calculate is different to the true mass.

Reply 8

Stonebridge

I agree. (#8)
The mass you should use is the given mass of the whole U238 nucleus.
This is quoted as 238.0507882 in Wiki
It should be provided in the question or on the data sheet.
You don't find the total mass of the U238 by adding the masses of its constituent protons and neutrons.
The total mass is less than the sum of the constituents. This is the basis of binding energy ('lost' mass) as #8 correctly states.
The same argument applies to the thorium.

Reply 9

Cora Lindsay

You need to use a consistent set of mass data. These come from the 1993 atomic mass evaluation published in Nuclear Physics A. Perhaps see if they work better?

The alphas emitted by U-238 are just under 4.2 MeV and the recoil energy will be about 0.08 MeV, so an answer around 4.3 MeV for the total decay energy seems sensible

Th-234 234.043956 amu

U-238 238.050784

He-4 4.002603