If the number of protons were 238, the mass of the nucleus would be
238 * 1836 * 9.1 * 10 ~ 31 = 3.976 * 10 ~ 25 kg.
If the number of protons were 239, the mass of the nucleus would be 239 * 1836 * 9.1 * 10 ~ 31 = 3.982 * 10 ~ 25 kg. Since the nucleus consists of protons and neutrons, the real mass of the nucleus lies between these estimated masses.
Answer: 3.980 kg
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