First slide

Radio activity

Question

The half life of a radio active substance is 2h. After 4 hr fractional change in activity will be

Moderate

A

$\frac{1}{2}$
B

$\frac{3}{4}$
C

$\frac{5}{8}$
D

$\frac{2}{3}$

Solution

(2)
$\begin{array}{l} A = A^{o} / {(2)}^{t / t 1 / 2} \\ A f t e r 4 h r, A = \frac{A^{o}}{{(2)}^{4 / 2}} = \frac{A^{o}}{4} \\ ∴ F r a c t i o n a l c h a n g e = \frac{A^{o} - A}{A^{o}} = 1 - \frac{1}{4} = \frac{3}{4} \end{array}$

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