Which solution has the higher osmolality(mmol/L)?
Normal Saline 19%
Most of you got it right but it seems that there was some of you thinking of Normal Saline as the higher osmolatlity. Lets break this down. Let's start with D5W. D5W contains 5gm of dextrose in 100ml or 50gm in a Liter. Since the Molecular weight is 180gm, the osmolality is 278mmol/L. This is isotonic to plasma.
Similarly, for D10W, it would turn out to be 556mmol/L. This is relatively hypertonic to plasma.
Normal Saline is 9gm of Nacl in 100ml or 90gm in a liter leading to an osmolality of 308mmol/L which is also isotonic. LR is also isotonic and has an osmolality of 274mmol/L.
So strictly in terms of OSMOLALITY, the highest is D10W.
When you give a dextrose based solution, regardless of the concentration or osmolality of the solution, the distribution of that solution in the body is equal in all compartments. Keeping the 60/40/20 rule in mind of water distribution, only 80 ml of a Liter of the dextrose solution remains in the intravascular space or plasma.
Compared to Normal Saline or LR, when you give a liter of it, 250cc of it remains in the plasma, making it more of a volume expander.
0.45% normal saline will be a mix of both ( 40cc of D5W and 125cc of Normal Saline ) leading to 165cc of it remaining in the plasma.
I would highly recommend the book by Dr. Joel Topf called The Fluid, Electrolyte and Acid-Base Companion to understand this concept better.
The basics of chemistry and electrolytes is simplified very well in his book. His blog is at http://www.pbfluids.com/ as well