15.29 b

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

Annie Lieu-1H
Posts: 68
Joined: Fri Sep 29, 2017 7:04 am
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15.29 b

I got part a of this question without using the answer solution method by simple logistics (A decreases at 1/3 rate of B because of coefficients so subtract that from initial A) but part b requires we use that method and I don't understand it.

Why do we substract (1molA/2molB) in the first place for [A]T since I thought that only referred to A concentration...

and beyond that.

thank you.

Brigitte Phung 1F
Posts: 50
Joined: Thu Jul 27, 2017 3:00 am
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Re: 15.29 b

So for part b) of this problem, we need to reuse the first-order integrated rate law again to solve for t. We need to convert from mols of B to mols of A and subtract this value from the initial A concentration to take into account how much A was used up to produce the 0.030 M concentration of B. This subtraction gives you the concentration of A at the time we're looking for! Hope this helps!