Hi...can anybody plz help me with the radioactive decay problem ib biology..?

Question

Potassium-40 decays to argon-40 with a half life of 1.3 billion years.When a volcano erupts and the molten rock cools, it contains 100% Potassium-40 and 0%argon-40 (because argon is a gas). A researcher found a fossil in volcanic rocks that contains 25%potassium-40 and 75%argon-40. How long ago did the fossilized organisms lived?

Answer 1

The half-life means that after that amount of time, half of the K-40 becomes Ar-40. After one half-life, you'd have 50% K and 50% Ar. After another half-life, half of the remaining K would change to Ar, giving you a total composition of only 25% K and 75% Ar, the situation described in the problem. Two half-lives is 2*1.3 billion = 2.6 billion years.

The strict way to solve this is to say log_2(100%/25%)*t, where log_2 means "log base 2" and t is the half-life. You'd need to use this for a situation where the factors of 2 weren't evident by inspection. In this case, it yields log_2(4)*t = 2t.

Answer 2

Decay of 100% to 25% takes two half-lives:

1 to 50% is one half-life and 50% to 25% is another half-life.

So, 2 * 1.3 billion years= 2.6 billion years

Done!

Answer 3

you can go to http://en.wikipedia.org...