Question

The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes `""_8^16X" and """_8^18X` in the sample?

Answer 1

It is given that the average atomic mass of the sample of element X is 16.2 u.

Therefore,

The atomic mass of the given element X is = 16.2 u.

Let `""_8^16 X` percentage = a

Then `""_8^18X` percentage = (100 − a)

Average mass of X = `16("a"/100) + 18((100 - "a")/100)`

16.2 = `((16"a")/100) + ((1800 - 18"a")/100)`

1620 = 1800 − 2a

1800 - 1620 = 2a

180 = 2a

a = `180/2` 

a = 90

Therefore `""_8^16X"` percentage = 90%

And `""_8^18X"` percentage = 100 − 90

= 10%