Just to make sure I'm on the same page as you are, you are doing the "Burning Calories: How Much Energy is Stored in Different Types of Food?" project, right? (URL: http://www.sciencebuddies.org/science-f ... p012.shtml
The amount of heat (energy) liberated by burning the food is
Q = m*c*(Tf-Ti)
where Q is energy in the form of heat, m is the mass of water in the container, c is the heat capacity of water, and (Tf-Ti) is the difference between the final and initial temperatures of the water. Only the energy in the part of the food that burned was released and (mostly) transferred to the water. That is why the pre- and post-test weights are so important--their difference tells you how much food actually burned.
mass of the food before being burned - mass of the food after being burned = mass of food burned
Let's say, for example, that you did an experiment burning a marshmallow and calculated that Q = 12,500 calories (with a little c). The weight of the marshmallow before the experiment was 6 grams; the weight of the marshmallow after the experiment was 3 grams. The mass of the marshmallow that burned was 8 grams - 4 grams = 4 grams. Here's the important part: if we want to know the energy released per gram of marshmallow, we need to divide Q by the mass of the marshmallow that burned:
energy content per gram of marshmallow = Q/mass of marshmallow burned
energy content per gram of marshmallow = 12,500 calories/4 grams
energy content per gram of marshmallow = 3,125 calories/gram
If I want to know the total energy content of the marshmallow, I multiple the energy content per gram of marshmallow by the pre-test weight of the marshmallow:
energy content of marshmallow = (energy content per gram of marshmallow)*(weight of marshmallow in grams)
energy content of marshmallow = (3125 calories/gram)*(8 grams) = 25,000 calories.
Let me know if you still have questions; I'm happy to help.