## 15.31 6th Edition

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

Janice Park 1E
Posts: 36
Joined: Fri Sep 28, 2018 12:22 am

### 15.31 6th Edition

For question 31 in Chapter 15 of the 6th Edition of the textbook, we are looking for the half life of iodine-131.
I know that we use the equation t(halflife) = 1/(k*initial concentration), but the answer key shows that t(halflife) is (1/k) * ln2.
Where does the ln2 come from? Is that supposed to be related to the initial concentration?

Sam Joslyn 1G
Posts: 34
Joined: Fri Sep 28, 2018 12:20 am

### Re: 15.31 6th Edition

ln2/k=t1/2 for first-order reactions, so I'm guessing the the decay of iodine-131 is a first-order reaction. It would follow that you use this equation, not the half-life equation for second-order reactions which is indeed t1/2=1/k[A]0

Alexa Tabakian 1A
Posts: 38
Joined: Fri Sep 28, 2018 12:20 am

### Re: 15.31 6th Edition

Make sure the order of the reaction corresponds to the right half-life equation.

harshitasarambale4I
Posts: 58
Joined: Fri Sep 28, 2018 12:26 am

### Re: 15.31 6th Edition

15.31 is not on the syllabus.