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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,

$204.4 = \frac{x \times 205 + (100 - x) \times 203.0}{100}$

$x = 70$

Hence, option B is correct.

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