Vaughan obtained the following data for the dimerization of butadiene at 326 ^◦C.

Time (min)        Partial pressure of butadiene (atm) 0 to be deduced
3.25........................................................0.7961
8.02........................................................0.7457
12.18......................................................0.7057
17.30......................................................0.6657
24.55......................................................0.6073
33.00......................................................0.5573
42.50......................................................0.5087
55.08......................................................0.4585
68.05......................................................0.4173
90.05......................................................0.3613
119.00....................................................0.3073
259.50.....................................................0.1711
373.00.....................................................0.1081

Determine whether the reaction is first, second, or third order. Find the rate constant, ignoring systematic errors. Find the initial pressure of butadiene.