Kinetics with Total Gas Pressures

[flybynight]

1. The problem statement, all variables and given/known data
I am considering the reaction 2C4H6 --> C8H12 at 320 degrees C. Both the product and the reactant are gases.
I have the data:
Time (minutes): 0.00 3.25 12.18 24.55 42.50 68.05
Total P (torr): 632.0 618.5 584.2 546.8 509.3 474.6

Find the reaction order and the rate constant.
Assume that only C4H6 is present at the start of the reaction.
2. Relevant equations
rate=k[C8H12]x
Perhaps PV=nRT?

3. The attempt at a solution
I tried to apply PV=nRT to the initial amount, hoping to find moles of reactant. However, I don't have a volume, so I don't know how to start. Any advice would be greatly appreciated.

[flybynight]

Any help?

[Borek]

Why does the pressure goes down? Can you use this change to calculate how much C4H6 reacted?

Your rate equation is wrong.

[flybynight]

I got it. Every time the reactant is changed into product, the pressure is one half for that molecule. So I found the values of the reactants and the products, and found the order is 1.

Thanks for the help,
Peter