Question

1. A granite rock is thought to be about 2 billion years old. why is it not possible to determine the age of the Rock using carbon-14 dating?

2. A hair sample has 80% of its original carbon-14 present. what is the age of the sample?

3. A bone fragment has lost 75% of its original carbon-14. what is the age of the bone fragment?

4. an organic sample is 28,650 years old. what percent of the original carbon-14 still present in the sample?

Answer 1

The half live of C14 is only about 6,000 years and this is not suitable to a rock sample of that age. There is a section is the link below that talks about which methods are best for various ages.
http://en.wikipedia.org/wiki/Radiometric_dating

2. k = 0.693/t_1/2
Look up t_1/2 in your text or notes and solve for k.
Then ln(No/N) = kt.
Set No = 100
N = 80
k from above
Solve for t.

#3 is the same process as #2.
4. ln(No/N) = kt
No = 100
N = x
k from above
t = 28,650 years.
Solve for N and convert to the percentage to the starting 100.

Answer 2

2 years