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It is not clear from your question if this is for a chemistry class or just for your own information. For a chemistry class, the solution can be written like this:
Explanation: 0.1 ppm by weight means that there is 0.1 gram of mercury (Hg) in 1 million (or 106) grams of fish. From the periodic table we find that one mole of mercury weighs 200.6 grams (this is the same value in grams as the atomic mass of mercury ). Finally, there are 6.02 × 1023 (Avogadro's number) atoms in one mole.
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