Question

A sample contains 10^–2 kg, each of two substances A and B with half lives 4 sec and 8 sec respectively. Their atomic masses are in the ratio 1 : 2. Amounts of A and B after an interval of 16 sec are respectively –

• A. 2.5 mg , 2.5 mg
• B. 0.625 mg , 2.5 mg
• C. 5 mg , 0.625 mg
• D. 5 mg , 2.5 mg

Answer 1

Answer: (B)

0.625 mg , 2.5 mg

Answer 2

For A : N_A = $\frac { N _ { 0 A } } { ( 2 ) ^ { 16 / 4 } }$ = = $\frac { 10 ^ { - 2 } } { 16 }$kg

= 0.625 mg

For B: N_B = $\frac { N _ { 0 B } } { ( 2 ) ^ { 16 / 8 } }$ = = $\frac { 10 ^ { - 2 } } { 4 }$ kg = 2.5 mg