Wich tools are appropriate to determine the densety of an irregular-shaped object ?

The most accurate way to determine an object's volume, especially in the case of an irregularly shaped object, is to immerse it in water and measure the amount of water it displaces. A graduated cylinder large enough to hold both the object and enough water to fully immerse it is the best tool for this job.

the temperature of the gas, in k, not celsius.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.