Fission of 1.00 kg of uranium-235 releases about 8.21 x 10^13 J, given 200 MeV per fission event. About how many uranium-235 atoms are in 1.00 kg?

1. One mole of uranium-235 has a mass of 235.04 g, so 1,000 g is 1,000 divided by 235.04, or about 4.25 mol. 2. Multiply the number of moles by Avogadro's number: 4.25 mol times 6.02 x 10^23 per mol. 3. Multiplying gives about 2.56 x 10^24 atoms. 4. 6.02 x 10^23 is simply Avogadro's number itself, before multiplying by the 4.25 mol. 5. 4.25 x 10^23 misplaces the decimal point on the mole count, and 2.35 x 10^25 overshoots by nearly a factor of ten. 6. So 2.56 x 10^24 atoms is correct. _Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.4 Nuclear Fission and Fusion_