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Chemistry LibreTexts

17.7: Osmotic Pressure

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Introduction

Semipermiable membranes do not let the solute pass through (Think of the sugar example). A solvent will move to the side that is more concentrated to try to make each side more similar! Since there is a flow of solvents, the height of each side changes, which is osmotic pressure. When we work with aqueous solutions, we use mm of H2O to describe the difference.

Osmosis is the diffusion of a fluid through a semipermeable membrane. When a semipermeable membrane (animal bladders, skins of fruits and vegetables) separates a solution from a solvent, then only solvent molecules are able to pass through the membrane. The osmotic pressure of a solution is the pressure difference needed to stop the flow of solvent across a semipermeable membrane. The osmotic pressure of a solution is proportional to the molar concentration of the solute particles in solution.

Π=inVRT=iMRT

where

  • Π is the osmotic pressure,
  • R is the ideal gas constant (0.0821 L atm / mol K),
  • T is the temperature in Kelvin,
  • i is the van 't Hoff factor
  • n is the number of moles of solute present,
  • V is the volume of the solution, and
  • M is the molar concentration of added solute (the i factor accounts for how many species in solution are generated)
Exercise 17.7.1

Calculate molarity of a sugar solution in water (300 K) has osmotic pressure of 3.00 atm.

Answer

Since it is sugar, we know it doesn't dissociate in water, so i is 1. Then we use Equation ??? directly

M=ΠRT=3.00atm(0.0821atm.L/mol.K)(300K)=0.122M

Exercise 17.7.2

Calculate osmotic pressure for 0.10 M Na3PO4 aqueous solution at 20°C.

Answer

Since Na3PO4 ionizes into four particles (3 Na+1 + PO34), then i=4. We can then calculate the osmotic pressure via Equation ???

Π=iMRT=(0.40)(0.0821)(293)=9.6atm

Exercise 17.7.3

Hemoglobin is a large molecule that carries oxygen in human blood. A water solution that contains 0.263 g of hemoglobin (Hb) in 10.0 mL of solution has an osmotic pressure of 7.51 torr at 25oC. What is the molar mass of the hemoglobin?

Answer

6.51×104g/mol


17.7: Osmotic Pressure is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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