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
If you understood this, you can solve b easily, right?

Last edited: Nov 8, 2012