# Homework Help: Half Lives and Radioactive Decay

1. Nov 8, 2012

### CharlieW95

1. The problem statement, all variables and given/known data

2. A meteor contains 6kg of molybdenum-93. Calculate:
a) The number of molybdenum molecules currently present in the meteor.
b) How long ago the meteor struck the earth if the original amount contained 20kg of molybdenum.

2. Relevant equations
2. a) 3.884x1022 molecules b) 6079.3 yrs

3. The attempt at a solution
Only topic i dont understand, exam tomorrow and i have no idea where else to go !

Any help is appreciated :)

(The answers were given, i need help working how to solve!)

Last edited: Nov 8, 2012
2. Nov 8, 2012

### Basic_Physics

Formulas:
N = No e-γt
where the decay constant, γ(gamma), and T1/2 are related by
γ$\:$T1/2 = ln(2)

3. Nov 8, 2012

### lep11

Well...all you need to do is solve for t.
3.
Now N=0.1*No
0.1*No = No*e-γt
0.1=e-γt
ln 0.1=-yt
t=(ln 0.1)/-y=(ln 0.1)/-(ln2/T_1/2)=(ln 0.1)/-(ln2/3.76x10^5 a)=1249044.964 a≈1.25 million years

4. Nov 8, 2012

### CharlieW95

Thank you both so much, understand 1 and 3 easily now. Question 2 is giving me grief!

5. Nov 8, 2012

### sjb-2812

Are you sure about a? 4 thousand molecules sounds very low.

6. Nov 8, 2012

### lep11

I am sorry, I didn't quite catch that.

7. Nov 8, 2012

### sjb-2812

The answer to a) has been given as above, which is about 4000 molecules.

8. Nov 8, 2012

### SHISHKABOB

copy pasting generally doesn't paste superscripts properly, so I would assume it's 1022

9. Nov 8, 2012

### sjb-2812

Maybe, but even taking that into account, still seems low.

10. Nov 8, 2012

### SHISHKABOB

3.884*1022 is low?

11. Nov 8, 2012

### sjb-2812

Yes. How many molecules are there in a mole

12. Nov 8, 2012

### lep11

Not sure if this is correct, but I got N=n*N_A=(m/M)*N_A=(6.0*10^3g/92.90681261g/mol)*6.0221327*10^23 1/mol=3.88914...*10^25