Math/Radioactive Dating
posted by Lena .
How would I calculate the age of a rock only given the amount of each in grams and halflife?
40K decays to 40Ar.
Half life of 40K is 1.25 billion years.
40K = 20 grams
40Ar= 5 grams
I think the formula is
Final Amount of 40K = Initial Amount of 40K * (1/2)^(time/halflife)
If I plug in the values I still don't have a value for time. And where would the grams of 40Ar fit in?

You started with 25 grams, you now have 20 grams
20=25(1/2)^(t/th)
.8=(1/2)^halflives
log both sides...
log (.8)=halflives*log.5
halflivees= log(.8)/log(.5)
time= 1.25billion* log(.8)/log(.5)
Respond to this Question
Similar Questions

Chemistry
Which radioactive sample would contain the greatest remaining mass of the radioactive isotope after 10 years? 
Geology
I need help with a radiometric dating equation. I am trying to determine the age of a sample that has a 40AR/40K ratio of 4.5. I understand the equation of D=Do+N(e^.693t1). I understand I would set Do to zero. But what is D? 
Chemistry
The age of a rock can be estimated by measuring the amount of 40Ar trapped inside. The calculation is based on the fact that 40K decays to 40Ar by a firstorder process. It also assumes that none of the 40Ar produced by the reaction … 
College Geology Homework
Suppose you have a sample of potassium feldspar from a rhyolite and you measure the ratio of atoms of 40Ar to 40K of 0.024 to 1, or for every 10000 atoms of 40K you find 240 atoms of 40Ar. For this problem, consider the decline of … 
Math/Radioactive Dating
My teacher said that if the ratio of mass (in a rock) for 40K and 40Ar was 1:3 then the age of the rock would be 2.5 billion years. I don't understand how he concluded that to be the age. The only thing he knew was that the halflife … 
phys
Although most natural potassium nuclei are stable 39K (93.3%) or 41K (6.7%) nuclei, about 0.0117% are radioactive 40K nuclei, which have a halflife of 1.26 billion years. What fraction of all the potassium nuclei in your body will … 
physics
Although most natural potassium nuclei are stable 39K (93.3%) or 41K (6.7%) nuclei, about 0.0117% are radioactive 40K nuclei, which have a halflife of 1.26 billion years. If you were worried about 40K radioactivity and wanted to wait … 
Geology
If the decay constant for radioactive decay of ^40 K is λ = 5.543*10^(10) / year, use this equation to calculate ages of the samples in the following table: t(1/2) = 0.693/λ 1)What is the half life of 40K? 
Physics  Half Life Question Check!!!
An ancient rock is found to contain 40Ar gas, indicating that 77% of the 40K in the rock has decayed since the rock solidified. Any argon would have boiled out of liquid rock. How long ago did the rock solidify? 
astronomy
The isotope potassium40 (40K) decays into the isotope argon40 (40Ar) with a halflife of 1.25 billion years. Suppose we find a rock in which seveneighths of the potassium40 has decayed to become argon40, and only oneeighth of …