Question

The activity per ml of a solution of radioactive substance is x. How much water be added to 200 ml of this solution so that the activity falls to x/20 per ml after 4 half-lives ?

• A. 100 ml
• B. 150 ml
• C. 80 ml
• D. 50 ml

Answer 1

Answer: (D)

50 ml

Answer 2

$\text{N = }\text{N}_{0}\left( \frac{1}{2} \right)^{n}\text{ }$

$\left( \frac{xv}{20} \right) = (x) \times 200\left( \frac{1}{2} \right)^{4}$

V = 250 ml.

So added volume of water = 50 ml.