Question

A student starts an experiment with 320 grams of a radioactive element Z and after 20 minutes he has left with 20 grams. Determine the half-life of element Z.

• A. 2 minutes
• B. 3 minutes
• C. 4 minutes
• D. 5 minutes
• E. 10 minutes

Answer 1

Answer: (D)

5 minutes

$t=\frac{2.303}{k}$$log\frac{R_o}{R}$
$20=\frac{2.303}{k}$$log\frac{320}{20}$
$k=\frac{2.303}{20}$$log\frac{320}{20}$
$k=0.115log16$
$k=0.14$
$t_{1/2}=\frac{0.693}{0.14}=4.95minute$
Hence, 5 minute is the answer.