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 by amount of solute and not by type of solute.When 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-?
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-.