Since water has a density of about 1g/mL, and we are assuming HCl and NaOH are mostly water, the total mass of the 2 solutions is 100 g. Due to the conservation of mass, the mass of product must equal the total mass of reactant, therefore the final mass must also be 100 g.
During the process of polymerization, monomers combine by sharing electrons. This process forms a polymer, which is made of repeating subunits. The resulting material is used in a variety of ways.
