## Formula 15.23

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

Christine Chen 1H
Posts: 64
Joined: Fri Sep 28, 2018 12:18 am

### Formula 15.23

Which formula/concept does this come from? This is problem 15.23 in edition 6
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Chloe Qiao 4C
Posts: 65
Joined: Fri Sep 28, 2018 12:27 am

### Re: Formula 15.23

I believe it is not come from a formula. I do not have the sixth edition, but it looks like that the question is asking the concentration A at the time when B is 0.034M. The part after the minus sign is to find how much A has been used to give rise to 0.034M B, and subtract that from 0.153M A(which I assume is the initial amount) gives the concentration of A at that time.

305113590
Posts: 66
Joined: Fri Sep 28, 2018 12:28 am

### Re: Formula 15.23

This is just stoichiometry. I think of it as this: since the concentration of B rises to 0.034M, I convert that into A using stoichiometry.

0.034(2mol A)=0.068M of A. What this means was that for the concentration of B to increase, 0.068M of A had to be used. Hence, 0.068M of A was lost from the initial 0.153M.

0.153-0.068=0.085M is the final A. After this, you would simply use the first order equation to find k