1. The problem statement, all variables and given/known data Why are Na and Cl the two most common ions? Why doesn't the ocean's salinity change 3. The attempt at a solution I would guess that they're the most abundant ions simply because there occur more often in nature than the other elements in sea water. As for the second part, I'd guess that rivers, precipitation along with evaporation, and minerals being eroded into the ocean tend to cancel one another out.