Osmotic Pressure Calculator

Calculate the osmotic pressure required to stop osmosis. Uses the van't Hoff equation π = iMRT for solutions.

Non-electrolyte=1, NaCl≈2, CaCl₂≈3

van't Hoff Equation: π = iMRT Where: • π = osmotic pressure (atm) • i = van't Hoff factor (number of ions) • M = molarity (mol/L) • R = 0.08206 L·atm/(mol·K) gas constant • T = temperature (Kelvin) van't Hoff Factor (i): • Non-electrolytes: i = 1 (glucose, urea) • NaCl (2 ions): i ≈ 1.9-2.0 • CaCl₂ (3 ions): i ≈ 2.6-3.0 • K₂SO₄ (3 ions): i ≈ 2.6-3.0
Example 1: Find osmotic pressure 0.1 M NaCl solution at 25°C i = 2 (Na⁺ + Cl⁻) M = 0.1 mol/L T = 298 K (25 + 273) R = 0.08206 L·atm/(mol·K) π = 2 × 0.1 × 0.08206 × 298 π = 4.89 atm Example 2: Find required molarity Target: 1.0 atm at 20°C (293 K) i = 1 (glucose, non-electrolyte) M = π / (iRT) = 1 / (1 × 0.08206 × 293) M = 0.0416 M

What is osmotic pressure?

Osmotic pressure (π) is the pressure required to stop the flow of solvent molecules through a semipermeable membrane from a dilute to a concentrated solution. It follows van't Hoff's equation: π = iMRT, where i is van't Hoff factor, M is molarity, R is gas constant, and T is temperature.

What is the van't Hoff factor (i)?

The van't Hoff factor (i) represents the number of particles a solute dissociates into in solution. For non-electrolytes: i = 1. For NaCl (dissociates into 2 ions): i ≈ 2. For CaCl₂ (dissociates into 3 ions): i ≈ 3. It accounts for ionic compounds producing more particles than expected from molar concentration.

How does temperature affect osmotic pressure?

Osmotic pressure is directly proportional to absolute temperature (in Kelvin). Doubling the temperature doubles the osmotic pressure. This is why refrigerated food lasts longer - lower temperature reduces osmotic pressure differences that cause spoilage. Always convert Celsius to Kelvin: K = °C + 273.15.

What is isotonic, hypertonic, and hypotonic?

Isotonic: equal osmotic pressure on both sides of membrane (π same). Hypertonic: higher osmotic pressure, water flows out (π higher). Hypotonic: lower osmotic pressure, water flows in (π lower). These terms are crucial in biology - red blood cells swell in hypotonic, shrink in hypertonic solutions.

Why is osmotic pressure important?

Osmotic pressure drives water transport in plants (transpiration pull), affects blood cell function in medicine, determines water balance in cells, is used in desalination, and is critical in food preservation (salt/sugar curing). It's also the principle behind reverse osmosis water purification.

What units are used for osmotic pressure?

Osmotic pressure can be expressed in: atm (standard), Pa (SI), mmHg (torr), bar, or psi. Conversion: 1 atm = 101,325 Pa = 760 mmHg = 1.01325 bar. For dilute solutions, the van't Hoff equation gives accurate results; for concentrated solutions, deviations occur.

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