Question

After three half-lives, a sample contains 20 grams of parent isotopes. How many grams of the parent isotope were present to start?

Answer 1

Three half lives meant that the mass was halved three times. Therefore, 20 * 2 * 2 * 2 to reverse the process yields 160 grams in the beginning.

Answer 2

Answer: 160 g

Explanation:

To calculate we use the formula:

`a=(a_o)/(2^n)`

where,

a = amount of reactant left after n-half lives = 20 g

`a_o` = Initial amount of the reactant = ?

n = number of half lives = 3

Putting values in above equation, we get:

`20g=(a_0)/(2^3)`

`20g=(a_0)/(8)`

`a_o=8* 20g=160g`