## 7B.3

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

Ellen Amico 2L
Posts: 101
Joined: Thu Sep 19, 2019 12:16 am

### 7B.3

How do you find the concentration of A in part c of this question?

It says:

Determine the rate constant for each of the following first- order reactions, in each case expressed for the rate of loss of A:
c) 2 A --> B + C
[A] initial = 0.153 M
and that after 115 s the concentration of B rises to 0.034 M.

nehashetty_2G
Posts: 102
Joined: Thu Jul 25, 2019 12:15 am

### Re: 7B.3

You just use stoichiometric relations to find the concentration of A. If 0.034M B was created, that means that two times that amount of A was used in creating B. Therefore, you subtract 0.068 from 0.153 to find [A]t (0.085M).

Niharika 1H
Posts: 50
Joined: Thu Jul 25, 2019 12:16 am

### Re: 7B.3

To find the concentration of A, you multiply (2molA/1molB) * (.034molB./L) to get 0.068 mol. Subtract this from the initial 0.153 to get 0.085