Question

How can I calculate the osmolarity of dextrose?

Answer 1

Osmolarity is a measure of the concentration of a solution in terms of the total osmoles (osm) of solute particles present in one liter of a solution. The SI unit of osmolarity is osm/L.
An osmole can be defined as the number of moles of a solute that contribute to the osmotic pressure of a solution.

Complete answer:
The osmolarity of a solution can be given by the formula
$osmolarity=\sum\limits_{i}{{{\varphi }_{i}}{{n}_{i}}{{C}_{i}}}$
Here, the osmotic coefficient $\varphi$ gives the degree of non-ideality of a solution. When there is a 100% dissociation, $\varphi$=1.
n is the number of ions that are formed upon dissociation,
C is the molar concentration of the solute, and
i is the index representing the identity of a solute.
Now, osmolarity is used to measure the osmoles of the solute particles. It is dependent on the property of a substance to dissociate.
Compounds that dissociate in a solution do not have a 1:1 ratio between osmolarity and molarity.
For example, ionic compounds such as NaCl salt dissociates into the solution like
$NaCl\to N{{a}^{+}}+O{{H}^{-}}$
So, 1 mole of a solution of NaCl dissociates into 2 osmoles of solute particles.
On the other hand, compounds that do not dissociate, like non-ionic compounds, have a 1:1 ratio between osmolarity and molarity i.e., one mol of solution forms 1 osmole.
We know that dextrose is a simple sugar and is chemically identical to D-glucose. It has the molecular formula ${{C}_{6}}{{H}_{12}}{{O}_{6}}$.

Since glucose is a non-ionic compound, it does not dissociate in an aqueous solution. Hence 1 mol solution of glucose forms 1osmol.
Similarly, 1 mol/L solution of dextrose is 1 osmol/L.

Note:
It must be noted that there can be multiple compounds that contribute to the osmolarity of a solution. It means that a N osm solution might consist of a moles of compound A, b moles of compound B, c moles of compound C, etc., and the sum of all the moles will be equal to n.
$a+b+c+.........=N$