Which Of The Following Generated Osmotic Pressure

Osmotic pressure, a colligative property, arises from differences in solute concentrations across a semipermeable membrane. Understanding which substances generate osmotic pressure requires grasping the fundamental principles governing this phenomenon. Let's delve into the key factors that determine a substance's ability to generate osmotic pressure, exploring different types of solutes and their behavior in solutions.

Defining Osmotic Pressure: The Basics

Osmotic pressure is defined as 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. This pressure is directly proportional to the molar concentration of the solute and the temperature of the solution, as described by the van't Hoff equation:

π = iMRT

Where:

• π is the osmotic pressure
• i is the van't Hoff factor (number of particles a solute dissociates into)
• M is the molar concentration of the solute
• R is the ideal gas constant
• T is the absolute temperature in Kelvin

This equation reveals that osmotic pressure depends on the number of solute particles in a solution, not their identity or size. This is why it's a colligative property – a property that depends on the concentration of solute particles, regardless of their nature.

Key Factors Influencing Osmotic Pressure Generation

Several factors determine whether a substance generates osmotic pressure. The primary considerations include:

1. Solubility: The substance must be soluble in the solvent to create a solution. Insoluble materials cannot contribute to osmotic pressure.
2. Dissociation: Substances that dissociate into ions in solution (electrolytes) generate a higher osmotic pressure than those that do not dissociate (non-electrolytes). The van't Hoff factor (i) accounts for this dissociation.
3. Membrane Permeability: The membrane must be permeable to the solvent but impermeable to the solute. If the solute can pass through the membrane, no osmotic pressure will be generated.
4. Concentration Gradient: A concentration difference of the solute across the membrane is essential. Without a concentration gradient, there is no net movement of solvent and thus no osmotic pressure.
5. Molecular Size: While osmotic pressure depends on the number of solute particles, very large molecules might exhibit deviations from ideal behavior due to steric effects or reduced activity.

Types of Solutes and Their Osmotic Pressure Contribution

To better understand which substances generate osmotic pressure, let's explore various types of solutes and their behaviors in aqueous solutions.

1. Electrolytes

Electrolytes are substances that dissociate into ions when dissolved in water, increasing the number of particles in the solution. This dissociation significantly contributes to osmotic pressure.

• Strong Electrolytes: These substances completely dissociate into ions. Examples include:

  • Sodium Chloride (NaCl): Dissociates into Na+ and Cl- ions. For NaCl, i ≈ 2.
  • Potassium Chloride (KCl): Dissociates into K+ and Cl- ions. For KCl, i ≈ 2.
  • Calcium Chloride (CaCl2): Dissociates into Ca2+ and 2Cl- ions. For CaCl2, i ≈ 3.
  • Magnesium Chloride (MgCl2): Dissociates into Mg2+ and 2Cl- ions. For MgCl2, i ≈ 3.
  • Sodium Bicarbonate (NaHCO3): Dissociates into Na+ and HCO3- ions. For NaHCO3, i ≈ 2.

  The osmotic pressure generated by these strong electrolytes is higher than that of non-electrolytes at the same molar concentration due to the increased number of particles.

• Weak Electrolytes: These substances only partially dissociate into ions. Examples include:

  • Acetic Acid (CH3COOH): Partially dissociates into H+ and CH3COO- ions. The van't Hoff factor (i) is between 1 and 2, depending on the degree of dissociation.
  • Ammonia (NH3): Reacts with water to form NH4+ and OH- ions, but the dissociation is limited. The van't Hoff factor (i) is close to 1.
  • Phosphoric Acid (H3PO4): Undergoes stepwise dissociation, releasing H+ ions in each step. The extent of dissociation varies depending on the pH of the solution.

  Weak electrolytes contribute less to osmotic pressure compared to strong electrolytes at the same molar concentration because they produce fewer ions in solution.

2. Non-Electrolytes

Non-electrolytes are substances that do not dissociate into ions when dissolved in water. These compounds dissolve as intact molecules.

• Sugars:

  • Glucose (C6H12O6): Dissolves in water but does not dissociate. The van't Hoff factor (i) is 1.
  • Sucrose (C12H22O11): Dissolves in water but does not dissociate. The van't Hoff factor (i) is 1.
  • Fructose (C6H12O6): Dissolves in water but does not dissociate. The van't Hoff factor (i) is 1.

  Sugars contribute to osmotic pressure proportionally to their molar concentration. Since they do not dissociate, their effect is less pronounced compared to strong electrolytes at the same concentration.

• Alcohols:

  • Ethanol (C2H5OH): Dissolves in water but does not dissociate. The van't Hoff factor (i) is 1.
  • Glycerol (C3H8O3): Dissolves in water but does not dissociate. The van't Hoff factor (i) is 1.

  Like sugars, alcohols contribute to osmotic pressure based on their molar concentration, without the amplification seen in electrolytes.

• Urea (CH4N2O): Dissolves in water but does not dissociate. The van't Hoff factor (i) is 1. Urea is a significant component of urine and contributes to the osmotic pressure of bodily fluids.

3. Colloids

Colloids are substances with particles larger than those in true solutions but small enough to remain dispersed in the solvent. They can contribute to osmotic pressure, but their effect is often less than that of smaller solutes at the same mass concentration.

• Proteins:

  • Albumin: A major protein in blood plasma. While proteins are large, they contribute to osmotic pressure, particularly in maintaining fluid balance in the body (oncotic pressure).
  • Globulins: Another class of proteins in blood plasma that contribute to osmotic pressure.

  The osmotic pressure generated by proteins is important in physiological systems, influencing fluid distribution between blood vessels and tissues.

• Starches:

  • Amylose and Amylopectin: Large carbohydrate molecules that can form colloidal dispersions in water. Their contribution to osmotic pressure is relatively small compared to simple sugars.

  Due to their large size, colloids have a lower molar concentration for a given mass concentration, resulting in a smaller osmotic effect compared to smaller molecules.

4. Polymers

Polymers are large molecules composed of repeating subunits. Their contribution to osmotic pressure depends on their molecular weight and concentration.

• Polyethylene Glycol (PEG): A synthetic polymer widely used in various applications. Its osmotic pressure contribution depends on its molecular weight and concentration.
• Dextrans: Polysaccharides used in intravenous fluids. They contribute to osmotic pressure and are used to expand blood volume.

Polymers typically have a lower osmotic effect per unit mass compared to smaller molecules due to their high molecular weight.

Factors Affecting the Magnitude of Osmotic Pressure

Several factors can influence the magnitude of osmotic pressure generated by a substance:

1. Temperature: As indicated by the van't Hoff equation, osmotic pressure is directly proportional to temperature. Higher temperatures increase the kinetic energy of the solute particles, leading to a higher osmotic pressure.
2. Solute Concentration: Osmotic pressure is directly proportional to the molar concentration of the solute. Higher concentrations result in greater osmotic pressure.
3. Ionic Strength: In solutions containing multiple ions, the ionic strength can affect the activity of the ions and, consequently, the osmotic pressure. The Debye-Hückel theory describes how ionic interactions influence the behavior of ions in solution.
4. Solvent Properties: The nature of the solvent can influence the solubility and behavior of the solute, indirectly affecting osmotic pressure. For example, the polarity of the solvent can affect the dissociation of electrolytes.
5. Real vs. Ideal Solutions: The van't Hoff equation assumes ideal solution behavior, where there are no interactions between solute particles. In real solutions, solute-solute interactions can deviate from ideality, affecting the observed osmotic pressure.
6. Van't Hoff Factor Correction: The van't Hoff factor, i, is a measure of the effect of a solute on colligative properties such as osmotic pressure. It is the ratio of moles of particles in solution to moles of solute dissolved. For ideal solutions, i is an integer equal to the number of ions formed per formula unit of solute. However, in real solutions, i may be less than the ideal value due to ion pairing in solution.

Examples of Osmotic Pressure in Biological and Industrial Systems

Osmotic pressure plays a critical role in various biological and industrial systems:

• Red Blood Cells: Red blood cells maintain their shape and function due to the osmotic balance between the intracellular fluid and the surrounding plasma. In hypotonic solutions (lower solute concentration), water enters the cells, causing them to swell and potentially burst (hemolysis). In hypertonic solutions (higher solute concentration), water leaves the cells, causing them to shrink (crenation).
• Plant Cells: Osmotic pressure (turgor pressure) is essential for maintaining the rigidity of plant cells. Water enters the cells, exerting pressure against the cell wall, which provides structural support to the plant.
• Kidney Function: The kidneys use osmotic pressure to regulate water balance in the body. The loop of Henle in the nephron creates a concentration gradient in the medulla of the kidney, allowing water to be reabsorbed from the filtrate.
• Reverse Osmosis: A water purification technique that uses high pressure to force water through a semipermeable membrane, separating it from dissolved solutes. This process is used to produce potable water from seawater or brackish water.
• Food Preservation: High concentrations of salt or sugar are used to preserve food by creating a hypertonic environment that inhibits the growth of microorganisms. Water is drawn out of the microbial cells, preventing their proliferation.
• Intravenous Fluids: Isotonic solutions, such as saline (0.9% NaCl), are used as intravenous fluids because they have the same osmotic pressure as blood plasma, preventing fluid shifts that could harm the patient.

Practical Examples: Calculating Osmotic Pressure

To illustrate how different substances generate osmotic pressure, let's consider a few practical examples:

Example 1: Sodium Chloride (NaCl)

Calculate the osmotic pressure of a 0.1 M solution of NaCl at 25°C (298 K).

• NaCl is a strong electrolyte that dissociates into Na+ and Cl- ions, so i = 2.
• R = 0.0821 L atm / (mol K)
• T = 298 K
• M = 0.1 mol/L

π = iMRT = (2)(0.1 mol/L)(0.0821 L atm / (mol K))(298 K) = 4.89 atm

Example 2: Glucose (C6H12O6)

Calculate the osmotic pressure of a 0.1 M solution of glucose at 25°C (298 K).

• Glucose is a non-electrolyte, so i = 1.
• R = 0.0821 L atm / (mol K)
• T = 298 K
• M = 0.1 mol/L

π = iMRT = (1)(0.1 mol/L)(0.0821 L atm / (mol K))(298 K) = 2.45 atm

Comparing these two examples, it is clear that NaCl generates approximately twice the osmotic pressure of glucose at the same molar concentration due to its dissociation into two ions.

Example 3: Acetic Acid (CH3COOH)

Calculate the osmotic pressure of a 0.1 M solution of acetic acid at 25°C (298 K), assuming it dissociates to 5%.

• Acetic acid is a weak electrolyte. If it dissociates to 5%, then α = 0.05 (degree of dissociation).
• CH3COOH ⇌ H+ + CH3COO-
• i = 1 + α(n-1) where n is the number of ions formed upon dissociation (n = 2 for acetic acid).
• i = 1 + 0.05(2-1) = 1.05
• R = 0.0821 L atm / (mol K)
• T = 298 K
• M = 0.1 mol/L

π = iMRT = (1.05)(0.1 mol/L)(0.0821 L atm / (mol K))(298 K) = 2.57 atm

Conclusion: Substances Generating Osmotic Pressure

In summary, osmotic pressure is generated by substances that:

1. Are soluble in the solvent.
2. Create a concentration gradient across a semipermeable membrane.
3. Increase the number of particles in solution, either through dissolution or dissociation.

Electrolytes, particularly strong electrolytes, generate higher osmotic pressures compared to non-electrolytes at the same molar concentration due to their dissociation into ions. Colloids and polymers contribute to osmotic pressure, but their effect is generally smaller than that of smaller molecules at the same mass concentration.

Understanding which substances generate osmotic pressure and the factors influencing its magnitude is crucial in various fields, including biology, medicine, environmental science, and industrial applications. The van't Hoff equation provides a valuable tool for quantifying osmotic pressure and predicting the behavior of solutions in different systems.