suppose a rock's chemical composition is analyzed and is found to contain 5x more potassium atoms than argon atoms. When did the rock form?

Question

A) 10 million years ago
B) 100 million years ago
C) 500 million years ago
D) 2 billion years ago
E) 4.5 billion years ago

Answer 1

Potassium-40 (40K) is a radioactive isotope of potassium which has a very long half-life of 1.251×109 years.

amount remaining=original amount*e^-.693 t/1.23e9
5x=6x*e^-.693 t/1.23e9
.833333=e^-.693 t/1.23e9
take ln of each side...
-0.18232=0.692t/1.23e9
t= 1.23e9*.833333/.693=1l48e9 years ago or 1.5 billion years ago.
answer D is the closest.

Answer 2

thanks so much! can you also use the formula
amount remaining= original amount (1/2)^ t/h when solving this question