## Quiz #2 Question #1 on Thursday

$\frac{d[R]}{dt}=-k[R]; \ln [R]=-kt + \ln [R]_{0}; t_{\frac{1}{2}}=\frac{0.693}{k}$

Mila 304269286
Posts: 17
Joined: Fri Sep 20, 2013 3:00 am

### Quiz #2 Question #1 on Thursday

Hey guys, I don't remember the exact question or how it was phrased, but on the second quiz, the first question asked you to find the time it took for a substance to reach earth, given that only 3 grams of the initial 700 g was present after the decay. Could someone let me know how to solve that problem? I had trouble with it. Thanks!

-Mila 1H

KVu 3G
Posts: 12
Joined: Fri Sep 25, 2015 3:00 am

### Re: Quiz #2 Question #1 on Thursday

You know that the substance is a first order, since the amount of the substance does not change the half life (50 days regardless of how much you start with.)

Since you know it's a first order, I believe you can simply keep dividing in half until you reach 3g OR

Use half life = 0.693/k and solve for k

Then use the first order rate law to solve for t. Even though you are not given molar mass to solve for concentration, you can still simply plug the mass into the eqn, since the molar mass should cancel on both sides of the equations through log rules.

($ln(Mass/Molar Mass) = ln(Mass) - ln(Molar Mass)$

Then the ln(Molar Mass) should cancel and you can solve for t. I hope this helped.