Potassium-40 undergoes radioactive decay to become argon-40. The half-life is 1.3 billion years. Which measurement would be most useful to find the absolute age of a rock that contains potassium?

Question

A.
the total mass of potassium-40 and argon-40

B.
the mass of argon-40

C.
the mass of potassium-40

D.
the ratio of potassium-40 to argon-40

Answer 1

To determine the absolute age of a rock that contains potassium-40 and argon-40, the most useful measurement would be:

D. the ratio of potassium-40 to argon-40

This ratio allows scientists to calculate how long the potassium-40 has been decaying into argon-40, using the known half-life of potassium-40. By comparing the amounts of both isotopes, one can deduce the age of the rock.