Calculation of percentage of isotopes — task. Chemistry CBSE Live product, Class 9 (2025-26).

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The average atomic mass of a sample of an element X is $16.2$ u. What are the percentages of isotopes $X^{16}$ and $X^{18}$ in the sample?

$\begin{array}{l} (\frac{16 \times x}{100}) + \frac{18 (100 - x)}{100} = 16.2 \\ \frac{16 x - 18 x + 1800}{100} = 16.2 \\ - 2 x + 1800 = 16.2 \times 100 \end{array}$

$x = 20$ and $(100 - x) = 80$
$x = 90$ and $(100 - x) = 10$
$x = 10$ and $(100 - x) = 90$
$x = 40$ and $(100 - x) = 60$

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4. Calculation of percentage of isotopes

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