Naturally occurring thallium consists of two stable isotopes, Tl-203 and Tl-205 (atomic masses = 203.0 and 205.0, respectively) and has an average atomic mass of 204.4. What is the percentage of Tl-205?

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Let us say we have `X%` of Ti-205. Then the rest `(100-X)%` is the Ti-203 percentage.

It is given that average atomic mass of Ti is 204.4.

Mass of Ti-205 in isotope` = X/100xx205`

Mass of Ti-203 in isotope `= (100-X)/100xx203`

`X/100xx205+(100-X)/100xx203 = 204.4`

`205X+20300-203X = 20440`

`X = 70`

*So the percentage of Ti-205 in the isotope is 70%.*

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