## HW 15.27

$K = \frac{k_{forward}}{k_{reverse}}$

Priscilla Okaiteye
Posts: 61
Joined: Tue Nov 21, 2017 3:02 am

### HW 15.27

A substance A decomposes in a first order reaction and its half-life is 355s. How much time must elapse for the concentration of A to decrease to :
a) 1/8[A]o

How would I go about starting to solve this question? Would I just use the first order half life equation?

deepto_mizan1H
Posts: 65
Joined: Fri Sep 28, 2018 12:16 am

### Re: HW 15.27

To start, you can use the half life for first order reaction like you said in order to find the rate constant k. Then, you can use another equation that describes the behavior of the reaction summed over time and plug in k and use ratios to find the value of t. You can also use a sample initial concentration of 10 M or anything to make the math easier.

You've got the right first step!

Elizabeth Gallmeister 1A
Posts: 41
Joined: Fri Sep 28, 2018 12:21 am

### Re: HW 15.27

Going off the earlier answer, once you find k, make sure you plug that into the right equation! Since this is a first order reaction, we know that ln[A]=−kt+ln[A]0. However, had it been a zero order or second order reaction, the equation would have been different.

Priscilla Okaiteye
Posts: 61
Joined: Tue Nov 21, 2017 3:02 am

### Re: HW 15.27

Oh, I get it now thanks, guys.