What would the mass of five grams of salt mixed with twenty grams of water be?
Despite Einstein, at this sort of speed and size, mass does not disappear. It is conserved
5 grams of anything mixed with 20 grams of anything is 25 grams of the mixture.
Now of course volume is not necessarily conserved, and the volumes would not necessarily add. (some of the salt would dissolve in the water so even thought the mass increased the volume might not.)