## practice quiz 1 number 9

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

Dahriel Aron 3A
Posts: 60
Joined: Fri Sep 25, 2015 3:00 am

### practice quiz 1 number 9

This question asks how long it takes the reactant concentration to drop from .0840 to .0220 with k= 8.39. I used ln final/initial = -kt and got .159 seconds but the answer is 4 seconds, and I was wondering what the right way to do this was?

DEVIKA CHANDRAMOHAN 1I
Posts: 12
Joined: Fri Sep 25, 2015 3:00 am

### Re: practice quiz 1 number 9

First, we have to determine what order the reaction is taking place in. This can be determined by looking at the units of k. For first order reactions, k is given in s-1. For second order, k is given in L mol-1 s-1. In this case, we have k in M-1 s-1 which is the same as L mol-1 s-1. Thus, we set this up as a second order reaction:

1/[A]t = 1/[A]0 + kt

When you plug in the values, you get 4 seconds. If you need to see the explanation for the units more clearly, see page 620 in the textbook. I hope this helps.