## Question 15.23

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

Posts: 53
Joined: Fri Sep 29, 2017 7:03 am

### Question 15.23

How do you solve for c? The question is determine the rate constant for each of the following 1st-order reactions, in each case expressed for the rate of loss
(c)2A-->B + C,given that[A]0 = 0.153mol L 1 and that after 115 s the concentration of B rises to 0.034 mol L 1.

Samantha Kan 2L
Posts: 30
Joined: Sat Jul 22, 2017 3:00 am

### Re: Question 15.23

You find the concentration of A used up, which is stoichiometry since you're given the concentration of B that rises. Then you subtract the concentration of A used up from the initial concentration of A for the final concentration. After that, since it's a first order reaction you use the equation ln[A] = -kt + ln[A]0 to solve for the rate constant, since you know t = 115 seconds.

Nha Dang 2I
Posts: 53
Joined: Fri Sep 29, 2017 7:07 am

### Re: Question 15.23

Why would you multiply [B] by 2 and not divide?

Xin He 2L
Posts: 38
Joined: Fri Sep 29, 2017 7:05 am

### Re: Question 15.23

You use stoichiometry to figure out how much of A is used up to form B. From the equation you gave, A should be used up half as fast as B is created, so you would multiply B by 2 and subtract it from A. Then you use the equation ln([A]/[A0]=-kt to find how much time has elapsed.