## 7B.13

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

Minh Ngo 4G
Posts: 137
Joined: Thu Jul 25, 2019 12:17 am

### 7B.13

For this question: The half-life for the second-order reaction of a substance A is 50.5 s when [A]0 0.84M . Calculate the time needed for the concentration of A to decrease to (a) one sixteenth; (b) one-fourth; (c) one-fi fth of its original value.
So for this question, I calculate k (0.0236) and then find eq for part a (15/0.84)/0.0236 and get 756s. The book value is 7.4x10^2 s.
What am I doing wrong?

Haley Dveirin 1E
Posts: 101
Joined: Sat Jul 20, 2019 12:17 am

### Re: 7B.13

I think the only difference is that in the book they round 0.0236 to 0.024 before plugging it into the last equation which slightly changes the answer from 756 seconds to 744 seconds (which using sig figs and scientific notation is 7.4 * 10^2).

HuyHa_2H
Posts: 100
Joined: Wed Sep 11, 2019 12:15 am

### Re: 7B.13

I believe they rounded at a certain point during the problem. I didn't round until the very end and got 756 seconds as well so you should be fine.

Delaney Smith 1C
Posts: 52
Joined: Thu Feb 28, 2019 12:15 am

### Re: 7B.13

Wait, how do you solve for k if you don't have [A]t?

Sally Qiu 2E
Posts: 105
Joined: Fri Aug 30, 2019 12:18 am

### Re: 7B.13

Delaney Smith 1C wrote:Wait, how do you solve for k if you don't have [A]t?

you're given the A naught and the half life so you can use the formula for the half life of a second order rxn in order to find k

Jessa Maheras 4F
Posts: 121
Joined: Fri Aug 02, 2019 12:16 am

### Re: 7B.13

Delaney Smith 1C wrote:Wait, how do you solve for k if you don't have [A]t?

Just use the other formula including k! The half life equation:)