HSC Question

[sidzeman]

Am i meant to use e = mc^2 in this - not really sure on how to do it

[pikachu975]

Hey!

Basically so we know plutonium 239 has an atomic number of 94 so there is 94 protons, 145 neutrons. Calculate the mass of these by using the conversion factor of 1 amu = 1.661x10^-27 kg.

Now they give you energy of the right hand side so you use 207.1 MeV and divide it by 931.5 to get the amu.

Now mass defect is what the question asks so do mass of CONSTITUENTS OF THE NUCLEUS minus the mass of the right hand side calculated above. Convert this to kg using 1.661 x 10^-27.

Correct me if I'm wrong, hope this helps.

[sidzeman]

Sorry where did you get the 931.5 which is divided from 207.1 MeV from? I don't do chemistry so this whole amu and all is new to me hahaha

[jamonwindeyer]

Am i meant to use e = mc^2 in this - not really sure on how to do it

You are, I believe! Pikachu's method definitely works but not quite what they'd intend for Physics; instead, the mass lost has to be linked to the energy released according to the equation you've said!

Convert energy to joules, then pop it in the equation to get your mass

[sidzeman]

wait so I did

207100(1.602 x 10^-19)

all over
(3 x 10^8)^2

But I'm still getting a slightly different answer am i doing something wrong?

[kiwiberry]

wait so I did

207100(1.602 x 10^-19)

all over
(3 x 10^8)^2

But I'm still getting a slightly different answer am i doing something wrong?

1MeV = 10⁶ eV, so it should be \( \frac{207.1 \times 10^6 \times (1.602 \times 10^{-19})}{(3 \times 10^8)^2}\)