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Q.

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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a

70.0 %

b

64.7 %

c

14.0 %

d

50.0 %

answer is B.

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Detailed Solution

Average atomic mass Is found as,

204.4=x×205+(100−x)×203.0100

x=70

Hence, option B is correct.

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