Kaplan General Chemistry Subject Test HELP!!!

[xaznxcountx]

Guys I really need your help for a question on the Kaplan Subject Test General Chemistry. Please help if you can explain why.

If you guys have a hold of the subject test it is question number 2 of the first Kaplan Subject test for GCHEM. The answer explanation is confusing and basically I DON'T UNDERSTAND THE ANSWER!

Here it goes:

For the reaction A + B -> C, determine the order of the reaction with respect to B from the information given below.

Initial [A] (M) Initial (M) Initial Rate of formation of C
2.5 x 10^-6 3 x 10 ^ -4 5 x 10 ^ -3
5 x 10 ^-6 3x 10 ^ -4 1x 10 ^ -2
1 x 10 ^ -5 9 x 10 ^-4 1.8 x 10 ^ -1

a. zero order
b. first order
c. second order
d. third order
e. fourth order

So the answer apparently is C which I really don't understand why...

Since we are talking about the initial concentrations of B, we look at the change from (in this case from 3 x 10 ^-4 to 9 x 10 ^ -4). Dividing them gives you 3. Okay chill, then we look at the rate formation in order to find the order of reaction. So 1.8 x 10 ^ -1 / 1 x 10 ^ -2 is 18. I don't understand how you get a second order from 3^(3) = 27. The explanation in Kaplan is more than useless so I'm not going to even bother with it. If ANYONE on this form can explain the answer to me that would be greatly appreciated.

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[gopher341238]

Your answer would be correct if the concentration of A was held constant during the change. A isn't held constant, however, so you need to take into account the change in concentration of A has on the rate. You need to first find that A is first order by comparing rows 1 & 2. Now you can compare rows two and three like you did. You see the change is 18 but A doubles from rows 2 and 3 so you have to divide 18/2=9 Now since B is tripled 3^2=9. You have your answer B is second order in this rxn.

 

[xaznxcountx]

O man I never knew that. Thanks! I hope I don't get one of these questions...