Ncert Exercises & Solutions

Chlorine exists in two isotopic forms, Cl-37 and Cl-35, but its atomic mass is 35.5. This indicates the ratio of Cl-37 and Cl-35 is approximately:

1. 1:2

2. 1:1

3. 1:3

4. 3:1

Hint: $average atomic mass = \frac{(Cl - 37 isotope % \times atomic mass) + (Cl - 35 isotope % + atomic mass)}{100}$

Step 1:

The fractional mass of chlorine is 35.5. The isotopic percentage of Cl-37 is X% and the isotopic percentage of Cl-35 is 100-x %.

average atomic mass = \frac{(Cl - 37 isotope % \times atomic mass) + (Cl - 35 isotope % + atomic mass)}{100}

Step 2:

Calculate the ratio is as follows:

35.5 = \frac{(X \times 37) + ((100 - X) \times 35)}{100}

3550 = 37 X + 3500 - 35 X

3550 - 3500 = 2 X

50 = 2 X

X = 25

100-X = 75

The ratio of Cl-37 and Cl-35 =

\frac{25}{75}

= 1:3