## 15.37

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

manasa933
Posts: 72
Joined: Fri Sep 29, 2017 7:04 am

### 15.37

In part (c), the mass is taken as the concentration because the vessel is sealed. But why is this true?

Grace Ramey 2K
Posts: 57
Joined: Thu Jul 27, 2017 3:01 am

### Re: 15.37

In this case, the mass can be used in place of the concentration because the conversion to concentration will cancel out in the first order integrated rate law:
ln[A] = -kt + ln[A]o
rewritten as ([A]/[A]o) = e^(-kt)

On the left-hand side, in [A]/[A]o, any conversion factors to mol/L will cancel out, so you can just use the numbers given in terms of grams.

Posts: 88
Joined: Fri Sep 29, 2017 7:03 am

### Re: 15.37

Why can we use the mass in this instead of concentration in this problem?

Laura Riccardelli
Posts: 49
Joined: Fri Sep 29, 2017 7:04 am

### Re: 15.37

I think you can use mass because you would be dividing by the same number of L on both sides so they would cancel anyways.