Biology: Solutions equilibrium

[future_vet]

Homework Statement

The solutions in the arms of a U-tube are separated at the bottom of the tube by a differentially permeable membrane. The membrane is permeable to sodium chloride but not to glucose. Side A is filled with a solution of 0.4 molar glucose and 0.5 molar sodium chloride (NaCl) and side B is filled with a solution containing 0.8 molar glucose and 0.4 molar sodium chloride. Initially the volume in both arms is the same. If you examine side A after three days, you should find...?

The Attempt at a Solution

The solution is: a decrease in the concentration of NaCl, an increase in the concentration of glucose and a decrease in the water level.

However, since the membrane is not permeable to glucose, why does the glucose level change? Why does the water level decrease?

Thanks!

 

[chemisttree]

You didn't mention which arm of the U tube would see a decrease in level.

I would think that since the membrane is permeable to NaCl that the concentration of NaCl in both sides of the U tube would equilibrate and there would be no osmotic pressure from that component. The glucose side would exert an osmotic pressure since it cannot cross the membrane but it is not clear, from the answer you gave, if the glucose side would see an increase in level or a decrease in level. You can figure that out, I'm sure.

 

[future_vet]

All the changes are in side A..

 

[Borek]

You have either changes of the solution level on both sides or on none. This is simple mass preservation.

 

[Moonbear]

However, since the membrane is not permeable to glucose, why does the glucose level change? Why does the water level decrease?

Let's try backing up a step in the thought process and see if these questions help you answer the above questions. Besides NaCl, what else in the tube can move through the membrane? And, how does that "other" substance moving through the membrane change if concentrations of either NaCl or glucose differ on either side of the membrane?