I'd say that it is incorrect because each carbon atom should bond 4 times and all three carbon atoms in this structural formula only bond once. The reverse is true for hydrogen atoms. They only form one bond but in this structural formula two of them have four bonds. I don't know about nitrogen though.
You can use the volume displacement method. If you had the alligator in a tank and you knew how much the volume of water in the tank was before the alligator was in there you could subtract that from the volume of the water with the alligator in the tank.
