Hypotonic, Hypertonic, and Isotonic Solutions Explained
In the Solutions chapter, one of the most frequently asked concepts is predicting the direction of solvent flow across a Semi-Permeable Membrane (SPM). To do this, we compare the Osmotic Pressure (π) of two different solutions.
Are the solutions Hypotonic, Hypertonic, or Isotonic? Let's break down the definitions and the mathematics behind them.
Video Tutorial: Predicting Solvent Flow
Watch Abhishek Sengar sir from CHEMCA expertly map out the relationships between Osmotic Pressure, Solute Concentration, and the resulting flow of the solvent.
The Mathematics of Osmotic Pressure
The osmotic pressure of a solution is directly proportional to its concentration and temperature. For electrolytic solutions, we must also include the Van 't Hoff Factor (i).
Where:
i = Van 't Hoff factor (number of particles after dissociation/association)
C = Molar Concentration (Molarity)
R = Universal Gas Constant
T = Temperature in Kelvin
Solvent molecules will ALWAYS flow from a region of Low Solute Concentration to a region of High Solute Concentration through a semi-permeable membrane. They want to "dilute" the more concentrated side!
The Three Cases (Comparing Solution A to Solution B)
-
Solution A is HYPOTONIC to B:
πA < πB (which means CA < CB).
Because Solution A has a lower concentration of solute, it has a higher concentration of water. Therefore, solvent flows FROM A to B.
Trick to remember: "Hypo" means low. The hypotonic solution gives away its water. -
Solution A is HYPERTONIC to B:
πA > πB (which means CA > CB).
Because Solution A is more concentrated, it is "thirsty" for water. Solvent flows FROM B to A.
Trick to remember: "Hyper" means high/excess. The hypertonic solution takes water in. -
Solution A is ISOTONIC with B:
πA = πB (which means CA = CB).
Because both solutions have the exact same effective concentration, there is no net flow of solvent across the membrane.
Fig: Visually tracking the concentration of solute dots allows you to instantly predict the flow of the blue solvent arrows.
Practice Questions for JEE & NEET
Let's apply these definitions to the two most common ways examiners test this concept!
Question 1 (Biological Application): Human Red Blood Cells (RBCs) are known to be isotonic with a 0.9% (m/v) NaCl solution. What will happen to an RBC if it is placed in a beaker containing a 2.5% NaCl solution?
Answer: The RBC will shrink (crenation).
Reasoning:
Inside the RBC, the concentration is equivalent to 0.9% NaCl. The solution outside the RBC is 2.5% NaCl.
Because the outside solution has a higher concentration, it is Hypertonic. According to our rules, solvent always flows from low concentration (inside the cell) to high concentration (outside the cell). As water leaves the RBC to dilute the salty solution outside, the cell shrinks and collapses.
Question 2 (The Van 't Hoff Trap): Solution A is a 0.1 Molar solution of NaCl. Solution B is an aqueous solution of Glucose (C6H12O6). If Solution A and Solution B are Isotonic at the same temperature, what must be the Molarity of Solution B?
Answer: 0.2 Molar
Reasoning:
As Abhishek Sir pointed out in the video, for Isotonic solutions, πA = πB.
Because R and T are constant, this means: iA × CA = iB × CB.
- NaCl is a strong electrolyte that dissociates into 2 ions (Na+ and Cl-). So, iA = 2.
- Glucose is a non-electrolyte (it does not dissociate). So, iB = 1.
Plugging into the formula:
2 × 0.1 = 1 × CB
CB = 0.2 M.
No comments:
Post a Comment