NCERT Question 11 - Chapter 4 Class 9 - Structure of Atom

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

Answer

Percentage of + = 100%

Let the percentage of be X%

And the percentage of be 100 - X%

We know the average atomic mass of a sample of an element X is 16.2 u.

Average mass of an isotope = (% existence of the isotope 1 x mass of isotope 1) + 

                                                    (% existence of the isotope 2 x mass of isotope 2)

(Mass of x X%) + [Mass of x (100-X%)] = 16.2 

[16 x (X/100)] + [18 x ( (100-X) /100 )] = 16.2

[16X] + [18 x (100 - X)] = 1620

[16X] + [1800 - (18X)] = 1620

1800 - 2X = 1620

2X = 180

X = 90

-> X = 90% and 100-X = 10%

That means 90% of and 10% of is present.