Question

What is the osmolarity of a solution with 136 mmol/L of sodium ions in units of mOsmo/L, assuming that chloride is available in the same solution at the same concentration?

Answer 1

An osmole is 1 mol of particles that contribute to 1 osmotic pressure.

As in given case NaCl solution as provided that same concentration of Cl- as Na+ have in solution.

For example:

NaCl dissociates as one mole of Na+ and one mole of Cl-.Hence total 2 moles are used in osmotic pressure.

NaCl --------->Na+(aq) + Cl-(aq) (1mol of each is used for osmotic pressure i.e. known as OSmol).

Here number of osmoles = 2.

In given case Na+ concenteration is 136mmol/L and also given that same for Cl-.

hence ,Osmolarity =136mmol/L * 2 Osmole=272mOsmo/L.

Conclusion : In general,whenever a NaX(X is any halogen) solution dissociates it will form two osmoles.

and general formula for OSmolarity = concenteration of one of ion into * 2Osmol