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?

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:

where,

a = amount of reactant left after n-half lives

= Initial amount of the reactant

n = number of half lives = 2

Putting values in above equation, we get:

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

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