Question

Two different solutions are separated by a semi-permeable membrane at 25 degrees Celsius. Solution A contains 0.30mM NaCl and 0.60mM CaCl₂ while solution B contains 1.00mM of glucose and 0.55mM of KBr. In which direction will water flow through the semi-permeable membrane?

Answer 1

Diffusion of solutes or solvent molecules across a semi permeable membrane depends on osmolar concentration of solutions. The unit for measuring osmolar concentration is milliosmole (mOsmol).It depends upon number of particles of the solution which in turn depends on the degree of dissociation of the solute

Osmolar concentration can be calculated as follows

Concentration of the solute x no. Of ions of the solute

Number of ions produced by   sodium chloride= 2

      So osmolar concentration of sodium chloride =.3 x 2 =.6 mOsmol/L

Number of ions produced by calcium chloride = 3

So osmolar concentration of calcium chloride =3x. 6 =1.8mOsmol/L

As glucose is not ionic, it's osmolar concentration= 1mOsmol/L

For KBr osmolar concentration = .55 x 2.=1.10 mOsmol/L

Solution A osmolar concentration is 1.8+.6 =2.4 mOsmol/L

Solution B osmolar concentration is 1+1.10 =2.10 mOsmol/L

As Osmosis leads to flow of water molecules from dilute solution to concentrated solution, here the direction of flow of water will be from solution B to solution A