Question

Lanthanum-138 has a half-life of 105 billion years. After 525 billion years, how much of a 240 g sample of this radioisotope will remain?

Answer 1

Answer:

15g is the answer.

Explanation:

Answer 2

Answer:

The answer is 7 g

Explanation:

given:

half life of Lanthanum - 138 is 105 Billion years.

let N be the initial amount.

525 billion years = 5 half lives

$N\ \frac{105 \ bn \ years}{}>\ \frac{N}{2}\ \frac{105 \ bn \ years}{}>\ \frac{N}{4}\ \frac{105 \ bn \ years}{}>\ \frac{N}{8} }\ \frac{105 \ bn \ years}{}>\ \frac{N}{16}\ \frac{105 \ bn \ years}{}>\ \frac{N}{32} }$

so, the remaining amount of the radioisotope = N/32

                                                                            = 240 / 32

                                                                            = 7.5 g