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  4. An element, X has the following isotopic composition; 200 X: 90 % 199 X: 8.0 % 202 X: 2.0 % The weighted average atomic mass of the naturally-occurring element X is closest to:

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Q. An element, $X$ has the following isotopic composition;
${ }^{200} X : 90 \%$
${ }^{199} X : 8.0 \%$
${ }^{202} X : 2.0 \%$
The weighted average atomic mass of the naturally-occurring element $X$ is closest to:

AIPMTAIPMT 2007Some Basic Concepts of Chemistry

Solution:

Weight of $^{200}X = 0.90 \times 200 = 180.00 \, u$
Weight of $^{199}X = 0.08 \times 199 = 15.92 \, u$
Weight of $^{202}X = 0.02 \times 202 = 4.04 \, u$
Total weight $ = 199.96 \approx 200 \, u$