1. For the simple decomposition reaction below, rate = k[AB]2 and k = 0.40 L/mols.

AB(g) A(g) + B(g)
If the initial concentration of AB is 1.60 M, what is [AB] after 13.0 s?

ANSWER:
(.4)(13)=5.2
1/1.6=.625
5.2-.625=4.575
1/4.575= .2185

I tried .22 & .21. They are both wrong. I have one try left. :( Please help me.

I don't think the right answer is listed.

I think it should be 1/(5.2+0.625) = ??
I think the answer that is supposed to be correct is the 1/4.575; however, that is the same as the third one but the reciprocal has not been taken for the number. I don't think 0.2185 is correct.