# Homework Help: Determining the rate law from experimental data

1. Sep 20, 2016

### fridakahlo

1. The problem statement, all variables and given/known data
First question on PDF:
http://ars-chemia.net/Classes/102/Problem_Sets/Practice Exam for Exam 1 Key.pdf

2. Relevant equations
I used the initial rates method:
Rate2/Rate1=k2*[A]^m/k1*[A]^m

3. The attempt at a solution
When I solve these types of problems with the initial rates method, I get multiple possible solutions. For instance, for reactant B, I got -2.5 when comparing rates 3 and 2, but when comparing rates 3 and 1 I got -0.5. I know the answer is -0.5.
My question is how do I know which one is the most plausible answer when solving these types of problems?

2. Sep 21, 2016

### epenguin

It was rather hard for me to make out what your question is. I guess it is about Question 1? This appears to be solved correctly, I presume the answers in red are yours? What is written above though does not correspond to this answer.

Last edited: Sep 22, 2016
3. Sep 29, 2016

### Dr Uma Sharma

you take any ratio ...answer should be same
If you take R1/R3 then [A] is cancelled and if you do R3 and R1 then plug in the conc of A with raise to power of its order .. it should give us the same answer ...