Question

Carbon-14 dating assumes that the carbon dioxide on Earth today has the same radioactive content as it did centuries ago. If this is true, the amount of 14C absorbed by a tree that grew several centuries ago should be the same as the amount of 14C absorbed by a tree growing today. A piece of ancient charcoal contains only 38% as much of the radioactive carbon as a piece of modern charcoal. How long ago was the tree burned to make the ancient charcoal? (The half-life of 14C is 5715 years. Round to the nearest year.)

Answer 1

7978 years

Explanation:

A = A02^(-t/hl)

where hl = half-life.

Dividing both sides by A0 and taking the logarithm, we get

ln(A/A0) = -(t/hl)ln2

or solving for t,

t= -(hl)ln(A/A0)/ln2

note that A/A0 = 0.38

t = -(5715 yrs)[ln(0.38)/ln2]

= 7978 years