Question

The average atomic mass of sample X is 80u. Find presentage of isotopes 79 35X and 81 35X in the sample

Answer 1

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}

100

x

× 16 + \frac{(100 - x)}{100}

100

(100−x)

× 18 = 16.2

\frac{16x}{100}

100

16x

+\frac{1800 - 18 x}{100}

100

1800−18x

= 16.2

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

100

16x+1800−18x

=16.2

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

100

−2x+1800

=16.2

-2x + 1800 = 16.2 × 100

-2x + 1800 = 1620

-2x = 1620 - 1800

-2x = -180

x = \frac{-180}{-2}

−2

−180

x = 90 %

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

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