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# Isotopes question

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1. A sample of gallium, Ar=69.7 was anysed and was found to consist of 65% Ga-69 and one other isotope. Determine the mass number of the other isotope in the sample of gallium
2. What have you tried so far?
3. (Original post by mydearestpotato)
A sample of gallium, Ar=69.7 was anysed and was found to consist of 65% Ga-69 and one other isotope. Determine the mass number of the other isotope in the sample of gallium
Lets say 1% = 1 atoms

so 100 atoms = 100%

Therefore total mass of 100 atoms = Ar x 100.

Therefore total mass of 100 atoms = (Y x M) + (Z x C).

Where, Y and Z are the mass of the isotopes, and M and C are the number of atoms (uses the fact that 1 atoms = 1%).

NOTE: Just remember Ar is the weighted mean mass of all the isotopes taking in account there abundance. So if you have 35-cl and 37-cl, the mean will not be 36? why because there might be more 35 than 37.

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