Question

Question 11 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?

Class 9 - Science - Structure of the Atom Page 55

Answer 1

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>> In order to solve this problem, we will have to suppose that the percentage of one of the isotopes in the sample is x , so that the percentage of the other isotope in the sample will be (100-x), Now :

(i) The mass of X isotope is 16 u. Suppose its percentage in the sample is x %.

(ii) The mass of X isotope is 18 u. Its percentage in the sample will be (100-x) %.

So , Average atomic mass of X = 16 × x/100 + 18 × (100-x)/100.

But the average atomic mass of X has been given to be 16 u. Therefore,

16.2 = 16 × x/100 + 18 × (100-x)/100.

16.2 = 16x + 1800 - 18x / 100

16.2 × 100 = 1800 - 2x

2x = 1800 - 1620

2x = 180

x = 180/2

x = 90

Thus the percentage of X in the sample is 90℅. The percentage of the other isotope X in the sample will be 100-90 = 10%.

Answer 2

This is chemistry question.

Que: Calculate the number of molecules of sulphur (S8) present in 16g of solid sulphur.

Answer: