Half Lives and Radioactive Decay

[CharlieW95]

Homework Statement

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.

Homework Equations

Answers:
2. a) 3.884x1022 molecules b) 6079.3 yrs

The Attempt at a Solution

Only topic i don't 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!)

 

[Basic_Physics]

Formulas:
N = N_o e^-γt
where the decay constant, γ(gamma), and T_1/2 are related by
γ$\:$T_1/2 = ln(2)

 

[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

 

[CharlieW95]

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

 

[sjb-2812]

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

 

[lep11]

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

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

 

[sjb-2812]

Homework Statement

...Answers:
2. a) 3.884x1022 molecules

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

 

[SHISHKABOB]

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

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

 

[sjb-2812]

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

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

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

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

 

[SHISHKABOB]

3.884*10²² is low?

 

[sjb-2812]

Yes. How many molecules are there in a mole

 

[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
,where N_A=Avogadro constant.
If you understood this, you can solve b easily, right?