Because 25% is half of 0.5 and the half life of 
is given as 5,715, the time it takes to reach this amount should just be double the half life: 11430 years.
However, in other cases the problem may not have easy numbers, so below is a more mathematical approach.
The half life formula is 
.
Solving for t gives:
Given information:
Plug in given values and solve for t.
With correct significant figures, this would be 11000 years.
K H D b d c m
So 10000 cm in a Km
A
Answer:
0.50 mol
Explanation:
The half-life is <em>the time required for the amount of a radioactive isotope to decay to half that amount</em>.
Initially, there are 8.0 moles.
- After 1 half-life, there remain 1/2 × 8.0 mol = 4.0 mol.
- After 2 half-lives, there remain 1/2 × 4.0 mol = 2.0 mol.
- After 3 half-lives, there remain 1/2 × 2.0 mol = 1.0 mol.
- After 4 half-lives, there remain 1/2 × 1.0 mol = 0.50 mol.
