The weight will vary depending upon the load and type of shot enclosed in the shells. A "typical" box of 25 cartridges containing 1 1/8 oz. (31.8 grams) of shot will weight right around 2 3/4lbs (1.25 kilograms.)
It depends on the size of the box to the density of the cereal. For example, a large box of Cheerios will weigh about 375g, but a small box of Swiss Muesli can way a kilogram.
You are not supposed to weigh any specific weight as a 12-year-old. The reason for this is that weight is related to height and body composition rather than age. For more information about weight in...