If we started with 70 mEq/L of NaCl, that would dissociate into 70 mEq/L of Na+ and 70 mEq/L of Cl-. That would make for a total of 140 mEq/L of solute dissolved in the water. Remember, osmotic pressure is independent of the identity of the solute. In other words, osmotic pressure is determined solely byWhen stating that 70 mEq/L of NaCl will dissociate into 140 mEq/L Na+ and Cl- ions, is that the total or each? 70 mEq/L of Na+ + 70mEq/L of Cl- = 140 mEq/L or 140 mEq/L Na+ and 140 mEq/L Cl-?

*amount*of solute and not by type of solute.

On this particular question, you could also think about conservation of mass/charge as a means to double-check your work. If there were 140 mEq/L of both Na+ and Cl-, we’d have summoned some Na+ and Cl- from nowhere. For example:

1 NaCl —> 1 Na+ + 1 Cl-

Thus, for every one NaCl, we get one Na+ and one Cl-.