Osmotic Pressure: Definition, Formula, Examples, Description, Types, Measurement

What is Osmotic Pressure?

Osmotic pressure is the minimum pressure that must be applied to a solution to stop the flow of solvent molecules through a semipermeable membrane—osmosis. It is colligative in nature and dependent on the concentration of the solute particles in the solution.

Formula of Osmotic Pressure

The Dutch chemist, Jacobus Van't Hoff proposed the relationship of the osmotic pressure. The Van't Hoff mathematical expression shows the relationship between osmotic pressure and the concentration of solutes. This mathematical relationship focuses on the fact that osmotic pressure is directly proportional to the number of solute particles in concentration.

π = iCRT

Where,

π is the osmotic pressure

i is the van't Hoff factor

C is the concentration of the solute in moles per unit volume of solution

R is the universal gas constant

T is the temperature

What Is Osmosis?

Osmosis can be defined as the flow of solvent molecules through a semipermeable membrane from a space with a low solute concentration to one with a high solute concentration. This occurs until equilibrium is achieved across the semipermeable membrane, with equal concentration on both sides.

The semipermeable membrane allows the movement of only solvent molecules and solute particles cannot pass through it. If sufficient pressure is applied to the solution side of the semipermeable membrane, the process of osmosis gets stopped. The minimum pressure that is required to just stop the process of osmosis is known as osmotic pressure.

Mechanism of Osmotic Pressure

Osmotic pressure develops because of the flow of solvent molecules from an area with a low concentration of solute to an area with a high concentration of solute to attain equilibrium. This movement then creates a pressure differential.

Osmotic pressure develops due to the movement of the solvent molecules from an area of low concentration of solutes to one with a high concentration of solutes to establish equilibrium. This move produces pressure, which should be balanced to halt the flow of a solvent.

Factors Affecting Osmotic Pressure

The factors affecting osmotic pressure are:

Solute Concentration: The higher the concentration of the solute, the higher the osmotic pressure.

Temperature: The kinetic energy of molecules increases with the temperature rise, and this, in turn, increases the osmotic pressure.

Nature of Solute: An ideal condition, however, exists because all solutes affect osmotic pressure differently due to variance in their dissociation in solution.

Concentration Gradient: A steep gradient in concentration across a semipermeable membrane, then high osmotic pressure would be produced.

Tonicity and Osmotic Pressure

Tonicity refers to the extent to which a solution causes a cell to swell or shrivel. It is determined by the osmotic pressure. It is the relative concentration of solutes in the solution outside a cell compared to that inside the cell.

Types of Solution

• Hypertonic: This is the solution where the concentration of solutes outside the cell is high due to which the cell loses water and shrinks.

• Hypotonic: A solution with a reduced concentration of solutes outside the cell, which means that it will gain water and swell.

• Isotonic: The concentrations of the solutes are equal inside and outside the cell; there is no net movement of water.

Effects on Cells

The effect on cells in different solutions are:

Osmotic Pressure in Plants

The osmotic pressure plays an important role in maintaining the structure and working of plants.

• Turgor Pressure: It is the pressure the cell membrane exerts on the cell wall and is facilitated by osmotic pressure. It helps in maintaining the posture of the plant upright.

• Plant Structure: Sufficient water supply that maintains cells in a turgid state, supports the upright position of the plant. A lack of water causes wilting due to a lack of turgor in the cells.

• Water Absorption by Roots: The roots of a plant absorb water from the soil due to osmosis, which forms part of the total water intake of the plant.

• Cell Expansion: Osmotic pressure allows the plant cells to take in water and expand. This can be associated with plant growth.

Osmotic Pressure in Human Physiology

Maintaining appropriate osmotic pressure is vital for human health.

• Homeostasis: Osmotic pressure regulates the balance of fluids inside and outside cells, ensuring proper cell function and homeostasis.

• Dialysis: In renal failure, dialysis machines use the principles of osmosis to remove waste products from the blood.

• Dehydration: If the consumption of water is low, then the osmotic pressure of the body fluids will be high. This causes the cell to lose water and shrink.

• Edema: Excess fluid in the body decreases the osmotic pressure in body tissues and leads to swelling.

Applications Of Osmotic Pressure

There are several applications of osmotic pressure in practical life.

Reverse Osmosis

• Water Purification: Because of the pressure exerted that is greater than the osmotic pressure, reverse osmosis removes the impurities from water, whether inorganic or organic compounds, by forcing the water molecules through a semipermeable membrane.

• Desalination: It makes seawater as fresh as water i.e., it makes seawater free from dissolved salts and other contaminants.

Medical Applications

• Intra Venous Fluid Administration: Intravenous fluids must always be isotonic to the blood to avoid disrupting the cells.

• Dialysis: In renal failure, dialysis machines use the principles of osmosis to remove waste products from the blood.

Food Preservation

• Water Reduction: It is through osmotic pressure that water is drawn out of food and in the process, microbial growth is reduced, allowing the food to be stored for a longer time.

Measurement of Osmotic Pressure

Osmotic pressure can be measured using special instruments and techniques.

• An osmometer is a device used to calculate the osmotic pressure either by measuring the freezing point depression of a solution or by vapour pressure.

• For accurate measurement, the temperature and concentration should be controlled very tightly.

Osmotic Pressure NEET MCQs (With Answers & Explanations)

The key concepts to be covered under this topic for different exams are:

• Factors affecting Osmotic pressure

• Types of Solutions

Practice Questions for NEET

Q1. The osmotic expansion of a cell kept in water is chiefly regulated by:

1. Mitochondria

2. Vacuoles

3. Plastids

4. Ribosomes

Correct answer: 2) Vacuoles

Explanation:

The osmotic expansion of cells in water is primarily regulated by distinct structures across various organisms. In plant cells, fungi, and bacteria, the cell wall serves a crucial role in this process. This rigid structure provides the necessary support and resistance against excessive water intake via osmosis, thereby averting lysis. Additionally, the cell wall in plant cells helps maintain turgor pressure as the vacuole enlarges, thus preserving structural integrity.

Conversely, animal cells, devoid of a cell wall, rely on the plasma membrane's active ion transport mechanisms to manage osmotic balance. These cells are vulnerable to bursting in hypotonic surroundings without such regulatory systems.

Hence, the correct answer is option 2) Vacuoles.

Q2. Osmotic pressure is minimum in __________ and highest in ____________

1. Hydrophytes, Xerophytes

2. Xerophytes, Hydrophytes

3. Mesophytes, Xerophytes

4. Hydrophytes, Mesophytes

Correct answer: 1) Hydrophytes, Xerophytes

Explanation:

Osmotic pressure is least in hydrophytes due to their aquatic habitat, which naturally entails lower solute levels inside their cells.

Conversely, xerophytes exhibit maximum osmotic pressure, as they inhabit arid regions and must effectively maintain elevated solute concentrations within their cells to efficiently attract and hold water from the environment.

Hence, the correct answer is option 1) Hydrophytes, Xerophytes

Q3. If the temperature is 300k and the concentration of solute is 0.5, what is the osmotic pressure?

1. 150R

2. 200R

3. 300R

4. 350R

Correct answer: 1) 150R

Explanation:

Osmotic pressure is the pressure required on the solution side to prevent the movement of solvent molecules across a semipermeable membrane toward the solution. It is equal and opposite to the force exerted by the solvent molecules and is defined as the excess hydrostatic pressure needed to equalize the water potential of the solution with that of pure water. Osmotic pressure, a colligative property, is measured using an osmometer and is represented by the symbol π (pi). It is a positive force and can be calculated using the formula:

π=iCRTπ = iCRT

Where:

• c= molar concentration

• i= ionization constant

• R = gas constant

• T = temperature

- wherein

c= molar concentration

i= ionisation constant

R= Gas constant

T= Temperature

OP = miRT (assume I=1)

= 0.5*1*300*R = 150R

Hence, the correct answer is option 1) 150R.

Also Read:

• MCQs on Osmotic Pressure

• Diffusion

• Symport, Antiport, Uniport

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