If one nucleus yields 200MeV=3.2⋅10−11J, then we need N=3.2⋅10−113.3⋅1010≈1.03⋅1021 of nuclei. The molar mass of U-235 is 235g/mol, where 1mol contains 6.02⋅1023 atoms/molecules. Therefore the mass of N U-235 nuclei (the mass of a nucleus is approximately the mass of an atom, as the electron mass is neglected) is 235⋅6.02⋅10231.03⋅1021≈0.4g.
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