Question

Half lives of two radioactive substances A and B are respectively 20 minutes and 40 minutes. Initially the sample of A and B have equal number of nuclei. After 80 minutes, the ratio of remaining number of A and B nuclei is –

• A. 1 : 16
• B. 4 : 1
• C. 1 : 4
• D. 1 : 1

Answer 1

Answer: (C)

1 : 4

Answer 2

A B

T_1/2 20 min 40 min

t = 0 N₀ N₀

t = 80 min N_A N_B

* n_A = 4 Ž N_A == $\frac { N _ { 0 } } { 16 }$

* n_B = 2 Ž N_B = = $\frac { \mathrm { N } _ { 0 } } { 4 }$

N_A : N_B = 1 : 4