Question

A 0.1 M NaCl solution is separated from pure solvent by a semipermeable membrane at STP as shown in the figure below –
An external pressure of 4 atm and 1 atm is acting on both the solution and solvent (including atmospheric pressure). Which of the following is correct about concentration of solution?

• A. Initially decreases and then become constant
• B. Initially increases and then become constant
• C. Continuously decreases
• D. Continuously increases

Answer 1

The correct option is A Initially decreases and then become constant

To prevent flow solvent particle from pure solvent side to solution side,
$p_1-p_2>\pi$ where $\pi$ is osmotic pressure solution.
At the beggining-
$P_1-P_2=3atm$
$\pi =iCRT=2\times 0.1\times 0.0821\times 300\approx 5atm$
$Thus{P}_1-P_2<\pi$
Hence solvent particle will flow from pure solvent side to solution side in the beginning. This will decrease the concentration level of solution initially,
After a while osmotic pressure will also decrease due to decreasing concentration. when osmotic pressure becomes 3 atm flow of solvent will stop and then after level will remains constant.