[Kax0r]

Natural uranium has an average mass of 238.02981 and contains the isotopes uranium-238, uranium-235 and uranium-234. The majority (99.28%) of all the atoms in natural uranium are uranium-238 (238.08amu). Most of the remaining atoms (0.72%) are uranium-235, and a slight trace of uranium-234. What is the atomic mass of uranium-235 (in AMU)?

[opti384]

The average mass of natural uranium is  238.02981. You are given that Uranium 238 exists as 99.28% in nature, which means that among 238.02981, 99.28% is from 238.08.

[igloo5080]

Treat this an an arithmetical calculation involving the concept of the average known as a 'mean'.

Assume that you have 100 uranium atoms in a typical sample.

99.28 will have a mass of 238.08 amu (you can find the total mass of these)

0.72 have a mass of x amu  (where x is the mass of the uranium-235 isotope) (once again, work out total mass)

Add these two masses together and divide by 100. This will equal the average mass given in the question.

Now solve this equation for x.

[Borek]

Weighted average.