Question

A semipermeable membrane separates two aqueous solutions at 20 ∘ C. For each of the cases, determine the direction, if any, of the net flow of water. Assume 100% dissociation for electrolytes, meaning ion pairing can be ignored.

Answer 1

Answer: 1. The total ion concentrations of both solution A and B are equal. Therefore, there is no net flow of water molecules.

2. The total ion concentrations of the solutions A is greater than that of solution B. Therefore, there is a net flow of water molecules towards A.

3. The total ion concentrations of the solutions B is greater than that of solution A. Therefore, there is a net flow of water molecules towards B.

Note: The question did not provide any reference solutions and their concentrations. Assuming the following pairs of solution and their given concentrations:

1. Solution A: 0.10M NaCl(aq) Solution B: 0.10M KBr (aq)

2. Solution A: 0.20M Al(NO3)3 solution B: 0.10M NaNO3

3. Solution A: 0.10M CaCl2 Solution B: 0.50M KCl

Explanation:

An electrolyte is a chemical substance that is decomposed when an electric current is passed through it either in its molten or solution form.

When there is 100% dissociation of electrolytes, the individual ions in the substance are free to move. Therefore, it is necessary to consider the concentration of ions instead of solution concentration.  The movement of water across the semipermeable membrane depends on the concentration of ions in each solution.This net movement of water molecules through a semipermeable membrane is from a low concentrated solution to the higher concentrated solution  and is known as osmosis .

First, calculate the concentrations of each solution A and B and indicate the movement of water molecules from low to high concentrated solutions as follows.

It is given that the electrolytes dissociate 100%(completely) into ions;

1)  Solution A: 0.10M NaCl(aq) Solution B: 0.10M KBr (aq)  

Calculate the concentration of ions in solutions A and B using,

Total ion concentration =  M o l a r i t y  ×  N umber o f  ions

Solution A:  Total ion concentration =  0.10 × 2 = 0.20M

Solution B:  Total ion concentration =  0.10 × 2 = 0.20M

The total ion concentrations of both solution A and B are equal. Therefore, there is no net flow of water molecules.

2)  Solution A: 0.20M Al(NO3)3 solution B: 0.10M NaNO3

Calculate the concentration of ions in solutions as follows ;

Total ion concentration =  M o l a r i t y  ×  N umber o f  i ons

Solution A:  Total ion concentration =  0.20 × 4 = 0.80M

Solution B:  Total ion concentration =  0.10 × 2 = 0.20M

The total ion concentrations of the solutions A is greater than that of solution B. Therefore, there is a net flow of water molecules towards A.

3)  Solution A: 0.10M CaCl2 Solution B: 0.50M KCl

Calculate the concentration of ions in solutions A and B using,

Total ion concentration =  M o l a r i t y  ×  N umber o f  i ons

Solution A:  Total ion concentration =  0.10 ×  3 = 0.30M

Solution B:   0.50 ×  2 = 1.0M

The total ion concentrations of the solutions B is greater than that of solution A. Therefore, there is a net flow of water molecules towards B.