Introduction to Osmolarity

(Review before Lecture 5)

 

1. Moles & Molarity 

 

Before beginning, review the concepts in “Introduction to Moles and Molarity”.

 

2. Osmolarity

 

Osmolarity is a measure of the osmotic pressure of a solution.  Osmolarity is measured in osmoles (Osm) / L of solution.  (1/1000 of an Osm is a milliosmole, or mOsm)   

 

Recall that the osmotic pressure of a solution is created by solutes dissolved in a solvent that creates a ‘pull’ on water INTO that solution.  

 

The more solute dissolved in a solution, the more osmotic pressure (and the higher the osmolarity).  So, a 5 Osm/L solution would have a higher osmotic pressure than a 2 Osm/L solution.

 

Thus, osmolarity is simply another measure of concentration, like molarity (see above).

 

However, osmolarity, rather than molarity, is used to indicate the number of particles produced by a solute when it is dissolved in a solvent (typically H2O).

 

IMPORTANT: The osmolarity of a solution depends upon the NUMBER, NOT THE TYPE, of particles dissolved in a solvent.  So, 1 particle (or mole or millimole)  of glucose would be equivalent to 1 particle (or mole, or millimole)  of Na, or Cl, or Mg, or anything else in solution.  The type of particle does NOT matter; only the NUMBER of particles contributes to osmolarity.

 

Covalently bonded molecules, e.g., glucose, do not break apart when dissolved in aqueous solvent).  However, ionically bonded compounds like NaCl, CaCl2, NaHCO3 do break apart into ions when dissolved in aqueous solutions:

 

NaCl → Na+ + Cl-

CaCl2 → Ca2+ + 2 Cl-

NaHCO3 → Na+ + HCO3-

 

IMPORTANT:  The # of osmoles in a solution is ALWAYS greater then, or equal to, the number of moles in a solution, i.e., # Osm ≥ # moles.

 

In order to calculate the osmolarity of a solution, simple take the number of moles in solution and MULTIPLY by the number of particles into which the dissolved compound dissociates.  So,

 

Osm/L = mol/L (this is molarity, M) x # particles yielded upon dissociation 

 

Example 1: The osmolarity of a 2 M NaCl solution would be:

 

Osm/L = 2 M NaCl x 2 particles (see dissociation equations above)  = 4 Osm/L

 

Example 2: The osmolarity of a 2 mM CaCl2 solution would be:

 

Osm/L = 2 mM CaCl2 x 3 particles (see dissociation equations above) = 6 mOsm/L

 

 

The osmolarity of human body fluids is about 290 mOsm/L.  Most of this (about 270 mOsm) is due to Na+, Cl-, and HCO3-. Other solutes that make contributions to this osmolarity include:  glucose, urea, plasma proteins, and other cations/anions.

 

3. Tonicity of body fluids

 

Tonicity is a measure of the difference in osmotic pressure between two solutions, e.g., the solution inside a cell vs. the fluid outside the cell.  So, tonicity deals with the relative osmotic pressures of different solutions.   

 

IMPORTANT: Water will always move from a solution with lower osmotic pressure into a solution with a higher osmotic pressure.  (Remember that there is more water in a solution with lower osmotic pressure, so it is just diffusing (osmosis) down its concentration gradient according to the law of diffusion.)

 

An isotonic solution is one in which two solutions have the same osmolarity (osmotic pressure) so, there is NO NET MOVEMENT OF WATER between the solutions.

 

Two important isotonic solutions you should commit to memory are:  0.9% NaCl and 5.0% glucose (dextrose).

 

A hypertonic solution is a solution that has a higher osmolarity (osmotic pressure) than a second solution (which would be hypotonic to it – see below): there is a net movement of water INTO the hypertonic solution (see the rule above about water always moving from lower osmotic to higher osmotic pressure).

 

A hypotonic solution is a solution that has a lower osmolarity (osmotic pressure) than a second solution (which would be hypertonic to it): there is a net movement of water OUT OF the hypotonic solution into the hypertonic solution (see the rule above about water always moving from lower osmotic to higher osmotic pressure). 

 

Example 1: What would happen if an erythrocyte (RBC) is placed in a hypertonic solution?   There would be a net movement of water from the hypotonic solution (inside of the RBC) OUT OF the RBC and the RBC would shrink (crenate). 

 

Example 2: What would happen if an erythrocyte (RBC) is placed in a hypotonic solution?   There would be a net movement of water from the hypotonic solution (outside of the RBC) INOT the RBC and the RBC would swell (and possibly lyse). 

 

 

Practice Problems:

 

1. How many moles are there in 150 g of the ionically bonded compound, KCl?

 

 

 

 

 

2. If you put the amount of KCl in question 1 in 2 L of H2O, what would be the molarity of the solution?

 

 

 

 

 

3. What would be the osmolarity of the solution in question 2?

 

 

 

 

 

4. A solution is made with 8.4 g of NaCl in 1 L of H2O.  Would this solution be isotonic, hypertonic, or hypotonic to human body fluids?