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The sample of the element chlorine consists of two isotopes, the isotope with mass 35 g/mol constitutes 24.4% of the sample and the isotope with mass 37 constitutes the remaining 75.6%. It can be seen that 24.4 + 75.6 = 100%, the two isotopes make up 100% of the sample's composition. For the mass of the two isotopes, the mass number that is given is used.
The average mass of the element is the weighted average mass of the two isotopes. This gives a mass of (0.244*35 + 0.756*37) = 36.512
The average mass of the chlorine sample is 36.512 g/mol
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