The osmotic pressure of a solution is primarily determined by the molar concentration of solute particles in the solution.
Understanding Osmotic Pressure
Osmotic pressure is a colligative property, meaning it depends on the number of solute particles in a solution, not their identity. It is the pressure required to prevent the flow of solvent across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. In simpler terms, it's the pressure needed to stop osmosis.
Factors Influencing Osmotic Pressure
While molar concentration is the primary determinant, other factors can also influence osmotic pressure:
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Molar Concentration (Molarity): As mentioned above, this is the key factor. A higher concentration of solute particles leads to a higher osmotic pressure. This relationship is described mathematically by the van't Hoff equation.
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Van't Hoff Factor (i): This factor accounts for the dissociation of ionic compounds in solution. For example, NaCl dissociates into two ions (Na+ and Cl-), so its van't Hoff factor is approximately 2. Glucose, a covalent compound, does not dissociate, and its van't Hoff factor is 1. The effective concentration, used in osmotic pressure calculations, is the molar concentration multiplied by the van't Hoff factor.
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Temperature (T): Osmotic pressure is directly proportional to the absolute temperature (in Kelvin). As temperature increases, the kinetic energy of the solute particles increases, leading to a higher osmotic pressure.
The Van't Hoff Equation
The relationship between osmotic pressure, concentration, temperature, and the van't Hoff factor is summarized by the van't Hoff equation:
Π = iMRT
Where:
- Π = Osmotic pressure
- i = van't Hoff factor
- M = Molar concentration (molarity)
- R = Ideal gas constant (0.0821 L atm / (mol K))
- T = Absolute temperature (in Kelvin)
Examples
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Example 1: Glucose Solution: A 1 M solution of glucose at 25°C will have a lower osmotic pressure than a 1 M solution of NaCl at the same temperature because glucose does not dissociate (i=1), while NaCl dissociates into two ions (i≈2).
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Example 2: Comparing Concentrations: A 2 M solution of glucose will have a higher osmotic pressure than a 1 M solution of glucose, assuming the same temperature.
Conclusion
In conclusion, the osmotic pressure of a solution is primarily determined by the molar concentration of solute particles, adjusted by the van't Hoff factor to account for dissociation, and directly proportional to the absolute temperature. The van't Hoff equation quantifies this relationship.
