Question

Say you have a rock composed entirely of potassium-40. After two half-lives of potassium-40, what fraction of the rock is still composed of potassium-40?

Answer 1

Answer: The fraction of the rock that is still composed of potassium-40 is 0.25

Explanation:

To calculate the fraction of the rock that is still composed of K-40, we use the formula:

$a=\frac{a_o}{2^n}$

where,

a = amount of reactant left after n-half lives

$a_o$ = Initial amount of the reactant

n = number of half lives = 2

Putting values in above equation, we get:

$\frac{a}{a_o}=\frac{1}{2^2}\\\\\frac{a}{a_o}=\frac{1}{4}=0.25$

Hence, the fraction of the rock that is still composed of potassium-40 is 0.25