Question

11. The average atomic mass of a sample of an element X is 16.2 u.
What are the percentages of isotopes X and X in the
sample?
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Answer 1

Explanation:

It is given that the average atomic mass of the sample of element X is 16.2 u. Let the percentage of isotope 8 18 X be y%. Thus, the percentage of isotope 8 16 X will be (100 − y)%. Therefore, the percentage of isotope 8 18 X is 10%.

Answer 2

Answer:

Lets take the percentage of the  isotope of  16 8 X as  --> x %

So , the percentage of the  isotope of 18 8X ---> (100 - x) %

We are given the average atomic mass as 16.2

\frac{x}{100}  × 16 +  \frac{(100 - x)}{100}  × 18 = 16.2

\frac{16x}{100} + \frac{1800 - 18 x}{100} = 16.2

\frac{16x + 1800 - 18 x}{100} = 16.2

\frac{-2x + 1800}{100} = 16.2

-2x + 1800 = 16.2 × 100

-2x + 1800 = 1620

-2x = 1620 - 1800

-2x = -180

x =  \frac{-180}{-2}

x = 90 %

Therefore , percentage of isotope of  16 8 X = 90 %

percentage of isotope of  18 8 X = 100 - x = 100 - 90 = 10 %

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