The density is the result to divide the mass to the volume.
In this case, the volume which is occupeted for the piece of silver is the difference between the volume with the silver inside the cylinder containing minus the initial volume (only water).
You HAVE TO know that molarity (M) tells you the number of moles of a solute per Liters of solution. M=mol/L 0.3M = 2.7 mol/ Volume Volume = 2.7 mol/3.0 L Volume = 0.9 L
There chemical reactions that can realise or absorb gas to the atmosphere. This means it is harder or impossible to measure it with normal weighing devices and normal taring equipment. As a principal in chemical reactions we have conservation of mass.