6.7
Osmotic Pressure
Osmosis is a process where solvent molecules move toward a solution through a semipermeable membrane. As the solution dilutes due to the entry of solv…
Osmosis is the movement of solvent across a semipermeable membrane toward a solution with a higher solute concentration.
As the solution dilutes and expands due to the added solvent, its hydrostatic pressure, which is the pressure exerted by a fluid at equilibrium due to gravity, increases, eventually halting osmosis.
Osmotic pressure, Π, is the pressure that needs to be exerted on a solution to prevent solvent influx.
The Π of an ideal solution is computed using the van't Hoff equation, which correlates Π with the solute's concentration.
Nonetheless, solutions of macromolecules like polymers are non-ideal due to excluded volume effects and polymer-polymer interactions. As a result, their molar masses, M, are calculated using the expanded van't Hoff equation involving the osmotic virial coefficient, B.
To simplify this equation, both sides are divided by the molar concentration of polymer J, which is the ratio of mass concentration, cmass,J to M.
Plotting Π/cmass,J versus cmass,J at various concentrations of J enables the estimation of the M value from the intercept and, then, the B value from the slope.
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Q1: What is osmotic pressure and how does it form?
Osmotic pressure, denoted Π, is the minimum pressure required to prevent solvent from moving across a semipermeable membrane into a solution. It develops as solvent molecules move toward higher solute concentrations, diluting the solution and increasing its hydrostatic pressure until equilibrium is reached and osmosis stops.
Q2: How does the van't Hoff equation calculate osmotic pressure?
The van't Hoff equation correlates osmotic pressure with solute concentration for ideal solutions, where solute-solvent interactions match those among solvent molecules. This equation provides a straightforward method to compute osmotic pressure from known concentrations, making it fundamental for understanding colligative properties.
Q3: Why do polymer solutions require a modified van't Hoff equation?
Polymer solutions are non-ideal due to excluded volume effects, where polymer chains cannot occupy certain spaces due to unfavorable overlapping, causing them to spread out more than ideal chains. Additionally, polymer-solvent interactions differ from solvent-solvent interactions, necessitating an expanded van't Hoff equation with the osmotic virial coefficient B.
Q4: How is polymer molar mass determined from osmotic pressure data?
By plotting osmotic pressure divided by mass concentration (Π/cmass,J) versus mass concentration (cmass,J) at various polymer concentrations, the molar mass M is determined from the y-intercept of the resulting line. The osmotic virial coefficient B is then calculated from the slope.
Q5: What role does hydrostatic pressure play in stopping osmosis?
As solvent enters the solution through the semipermeable membrane, the solution expands and its hydrostatic pressure increases. When hydrostatic pressure equals osmotic pressure, the driving force for solvent movement ceases, halting osmosis and establishing equilibrium across the membrane between solutions.
Q6: What is excluded volume effect in polymer solutions?
Excluded volume effect describes the space a polymer chain cannot occupy due to unfavorable chain overlapping. This causes polymer molecules to be more spread out than ideal polymer chains would be, affecting their osmotic behavior and requiring corrections to the van't Hoff equation for accurate molar mass calculations.
Q7: How does osmosis differ between ideal and nonideal two component liquid solutions?
Ideal solutions follow the van't Hoff equation directly because solute-solvent interactions match solvent-solvent interactions. Nonideal two component liquid solutions like polymer systems deviate due to excluded volume effects and different intermolecular interactions, requiring the expanded van't Hoff equation with the osmotic virial coefficient for accurate pressure calculations.