Question

Suppose a salt and a glucose solution are separated by a membrane that is permeable to water but not to the solutes. the nacl solution has a concentration of 1.95 g per 250 ml of water (molecular weight = 58.5). the glucose solution has a concentration of 9.0 g per 250 ml of water (molecular weight = 180). calculate the molality, millimolality, and milliosmolality of both solutions. state whether osmosis will occur and, if it will, in which direction. explain your answer.

Answer 1

1)

m(NaCl) = 1.95 g
V(H2O) = 250mL
M(NaCl) = 58.5 g/mole

Since waters density value is 1g/mL, it can be assumed that volume and mass of water are same values:

V(H2O) = 250ml = 250g = 0.25 kg

molality of NaCl:

n(NaCl)=m/M=1.95/58.5= 0.033 mole

molality b(NaCl)=n(NaCl) / V (H2O)= 0.033/0.25 = 0.132 mol/kg

milimolality of NaOH = 0.132/0,001 = 132 mmole/kg

milliosmolality of NaOH = milimolality x N of ions formed in dissociation

Since NaCl dissociates into 2 ions in solution:
                                        
milliosmolality of NaOH = 132 x 2 = 264  osmoles/kg

2)

m(gl) = 9 g
V(H2O) = 250mL
M(NaCl) = 180 g/mole

Since waters density value is 1g/mL, it can be assumed that volume and mass of water are same values:

V(H2O) = 250ml = 250g = 0.25 kg

molality of glucose:

n(gl)=m/M=9/180= 0.05 mole

molality b(gl)=n(gl) / V (H2O)= 0.05/0.25 = 0.2 mol/kg

milimolality of glucose = 0.132/0,001 = 200 mmole/kg

milliosmolality of glucose = milimolality x N of ions formed in dissociation

Since glucose does not dissociate, milimolality and milliosmolality are same:
                                        
milliosmolality of glucose = 200 osmoles/kg

3)

The osmosis represents the diffusion of solvent molecules through a semi-permeable membrane that allows passage solvent molecules but does not to the dissolved substance molecule. The osmosis occurs when the concentrations of the solution on both sides of the membrane are different. Since the semi-permeable membrane only permeates the solvent molecules, but not the particles of the dissolved substance, it occurs the solvent diffusion through the membrane, i.e. the solvent molecules pass through the membrane to equalize the concentration on both sides of the membrane. Solvents molecules move from the middle with a lower concentration in the middle with a higher concentration of dissolved substances.

In our case, osmosis will occur because the concentration of NaCl solution and the concentration of glucose solution do not have same values. Osmosis will occur in the direction of glucose solution because it has a lower concentration.