📘 Osmotic Pressure
Osmotic pressure (π) is the minimum external pressure that must be applied to a solution to stop the flow of solvent molecules through a semipermeable membrane.
For molar mass determination:
Therefore,
where w is the mass of solute, M is its molar mass, V is the volume of solution, R is the gas constant and T is the absolute temperature.
🔬 Why is it better for proteins and other biomolecules?
Osmotic pressure is particularly suitable for determining the molar mass of biomolecules such as proteins, enzymes and polymers because these substances generally have very high molar masses and may decompose when heated.
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Heat Sensitive
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Dilute Solutions
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Osmotic Pressure Method is Preferred
1. Suitable for Very High Molar Mass
For a given mass of a high-molar-mass biomolecule, the number of moles is very small. Other colligative properties such as elevation in boiling point or depression in freezing point may therefore be extremely small and difficult to measure accurately.
Osmotic pressure can be measured even for very dilute solutions, making it more sensitive for such substances.
2. No Heating is Required
Proteins and other biomolecules are often heat-sensitive. Methods involving boiling or freezing may cause decomposition, denaturation or structural changes.
Osmotic pressure can be determined at or near room temperature, so the biomolecule is less likely to undergo thermal decomposition.
3. Suitable for Dilute Solutions
Biomolecules can often be studied only in very dilute solutions. Osmotic pressure is directly proportional to the concentration:
Thus, even a dilute solution gives a measurable osmotic pressure.
4. Direct Relation with Molar Mass
For a non-electrolyte:
Rearranging:
Hence, the molar mass can be calculated directly from the measured osmotic pressure.
✅ Final Answer
Osmotic pressure is preferred for determining the molar mass of biomolecules such as proteins because it can be measured at room temperature using very dilute solutions.
Proteins have very high molar masses, so their other colligative effects such as elevation in boiling point and depression in freezing point are usually too small to measure accurately. Moreover, heating can denature or decompose proteins. Osmotic pressure avoids these problems and therefore gives a more reliable molar mass.
1. Osmotic pressure is a:
✅ Answer
A) Colligative property
2. The equation for osmotic pressure of a dilute solution is:
✅ Answer
A) π = CRT
3. Osmotic pressure is especially useful for determining the molar mass of:
✅ Answer
B) Proteins and polymers
4. Proteins are preferably studied by osmotic pressure because:
✅ Answer
B) They are often heat-sensitive
5. Which process may damage proteins?
✅ Answer
A) Heating
6. Osmotic pressure can be measured using:
✅ Answer
B) Dilute solutions
7. The molar mass calculated using osmotic pressure is:
✅ Answer
B) M = wRT/πV
8. Osmotic pressure is measured across a:
✅ Answer
B) Semipermeable membrane
9. For a dilute solution, osmotic pressure is proportional to:
✅ Answer
C) Both A and B
10. Which property is generally very small for high-molar-mass proteins?
✅ Answer
B) Elevation in boiling point
11. Another name for osmotic pressure equation π = CRT is:
✅ Answer
B) van't Hoff equation
12. Osmotic pressure is measured at:
✅ Answer
C
13. Proteins generally have:
✅ Answer
B) Very high molar masses
14. Which property does not require boiling the solution?
✅ Answer
A) Osmotic pressure
15. The unit of osmotic pressure can be:
✅ Answer
A) atm
16. Osmotic pressure depends mainly on the:
✅ Answer
A) Number of solute particles
17. Which is an advantage of osmotic pressure for biomolecules?
✅ Answer
B
18. The molar mass of a solute increases when, keeping w, T and V constant, osmotic pressure:
✅ Answer
B) Decreases
19. Osmotic pressure is a property of:
✅ Answer
B) Solutions
20. The best reason for using osmotic pressure for proteins is:
✅ Answer
B
Q. Why is osmotic pressure preferred for determining the molar mass of proteins?
Answer: Proteins have very high molar masses and are often heat-sensitive. Osmotic pressure can be measured at relatively low temperature using very dilute solutions, whereas other colligative effects may be too small to measure accurately.
Q. Give three reasons why osmotic pressure is suitable for determining the molar mass of biomolecules.
- Biomolecules have very high molar masses.
- Osmotic pressure can be measured using dilute solutions.
- No heating or boiling is required, reducing the possibility of denaturation or decomposition.
Q. Derive the expression used to determine the molar mass of a biomolecule from osmotic pressure.
For a dilute solution:
Since concentration:
Therefore:
Rearranging:
Thus, the molar mass can be calculated from the mass, volume, temperature and measured osmotic pressure.
Q. Explain why osmotic pressure is a better method than elevation in boiling point or depression in freezing point for determining the molar mass of proteins.
Solution:
- Proteins and other biomolecules generally possess very high molar masses.
- For a given mass, their number of moles is very small.
- Consequently, their elevation in boiling point and depression in freezing point are extremely small and difficult to measure accurately.
- Osmotic pressure can be measured even in very dilute solutions.
- The measurement can be performed without heating, so heat-sensitive proteins are less likely to denature or decompose.
Q. Explain in detail why osmotic pressure is preferred for determining the molar mass of biomolecules such as proteins.
1. High Molar Mass
Proteins, enzymes and many polymers have very high molar masses. Therefore, even when a measurable mass is taken, the number of moles present is small.
2. Very Small Other Colligative Effects
The elevation in boiling point and depression in freezing point depend on the molality of the solution. For high-molar-mass substances, the molality is often extremely small, giving very small temperature changes.
3. Osmotic Pressure is Measurable in Dilute Solutions
Even a dilute solution can produce a measurable osmotic pressure. This makes the method suitable for biomolecules that are commonly studied in dilute solutions.
4. No Heating Required
Many proteins are heat-sensitive and may undergo denaturation or decomposition at elevated temperatures. Osmotic pressure can be measured without boiling the solution.
5. Direct Calculation of Molar Mass
M = wRT/(πV)
Thus, the molar mass can be calculated directly from the experimentally measured osmotic pressure.
Osmotic pressure is preferred because it is sensitive for dilute solutions, can be measured without heating, and is therefore especially suitable for high-molar-mass, heat-sensitive biomolecules such as proteins.
🎯 Quick Revision
Molar mass: M = wRT/(πV)
Proteins: High molar mass + heat-sensitive
Solution: Very dilute solution can be used
Temperature: No boiling/heating required
Result: More reliable molar-mass determination
