Pressure in Popcorn - Lab

Last Friday, we had a chemistry lab all about popcorn. We were given the task to calculate the pressure (ATM) of  popcorn kernels through the popping process. All kernel have some water in them as wheel. We started with around 40 kernels and measured the volume by displacing water. Then we poured a small amount of cooking oil into a beaker and then followed up by placing our kernels in after. We proceeded to use a gas burner to heat up the bottom of the beaker until most of the Kernel were popped without burning the popped ones to ash. We weighed the beaker before and after and used all of the data together to get our answer. 

Stats:

Initial Volume: 4.9 ml

New Volume:  10.4 mL

40 kernels

37 kernels

Pre Pop Mass: 111.21  g

Post Pop Mass: 109.54 g

225 degrees Celsius

Math:

5.5 ml

111.21 / 109.54

1.67 x 1 / 18 = 0.093 mole

P = nRT / v 

(0.093)(0.0821)(498) / .0055 = 691.3 / 37

18.68 ATM

I am guessing all of the kernels did not pop because they all did not have the same amount of surface area being burned at the same time. Time was crucial because we could not let the quick popping pop corn burn while we waited for un-popped kernels to pop. Because all of the kernels did not pop this could have affected the weight before and after. This could have skewed the data slightly.