Why dosn't the density of an object change when you adjust the volume and mass?

Density decreases when volume increases if mass is kept constant.

Explanation: For example, if an iron rod is heated, it will get longer and fatter and its density will decrease. This happens because the mass of the rod stays the same, but its volume increases. ... When cooled down, most matter decreases in volume and increases in density.

Explanation:  Density is the ratio of the mass of substance to the volume.

When mass (m) of the substance increases, density will increase as it is directly proportional to the mass and when the volume (V) of any substance is decreases then the density increases because the volume and density have inverse relationship.

                                                   D = m/V

If the mass and the volume of the same substance gets changes by the same proportion like if the increase in the mass and decrease in the volume has same quantity then there will be no change in the value of the density.

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.