HC1 solution is mostly water. NaOH solution is mostly water. When 50mL of HC1 is mixed with 50mL of NaOH, what is the final mass?

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.