Osmotic Pressure

What is Osmotic Pressure?

Osmotic pressure is a minimum pressure that is supposed to be applied to a solution to halt the incoming flow of its pure solvent across a semipermeable membrane (osmosis). It is basically a colligative property and is purely dependent on the concentration of solute particles of the solution.

Jacobus, a Dutch chemist, found a quantitative relationship between the osmotic pressure and solute concentration, expressed the following as an Osmotic Pressure equation.



  • ‘π’ is the osmotic pressure

  • i is dimensionless van ‘t Hoff index

  • c is the molecular concentration of solute in the solution

  • R is the ideal gas constant

  • T is the temperature in kelvins

Furthermore, it is essential to note that the derived Osmotic Pressure equation holds only true for solutions that behave the same as ideal solutions. Osmotic pressure of pure water is 0 because it has 0 osmotic pressure.

Understanding the Osmotic Pressure

Consider a U-Tube showing an osmotic pressure diagram below which is known as the Osmotic Pressure diagram.

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The left side of the U-tube contains an aqueous solution, and the right side is of pure water. Here, the pure water is trying to dilute the solution by passing through the semipermeable membrane. Eventually, the weight added of the excess water on the left tube causes enough pressure to halt osmosis.

As we discussed, the Osmotic pressure is the one that needs to be applied to a solution to prevent an inward solution to prevent the inward flow of water across a semipermeable membrane. It is also explained as the pressure required to nullify osmosis. One of the ways to stop osmosis is to increase the hydrostatic pressure on the solution side of the membrane. This ultimately squeezes the solvent molecules together closer, increasing their “escaping tendency.” Whereas, the escaping tendency of a solution can be raised until it becomes equal to the molecules in the pure solvent. And at this point, osmosis will cease. Osmotic pressure is the one required to achieve osmotic equilibrium.

Osmotic Pressure Example

How much glucose (C6H12O6) per liter should we use for an intravenous solution to match the 7.65 atm at 37 degrees Celsius osmotic pressure of blood?


The Osmotic Pressure calculation example is given with a brief description below.

Osmotic pressure is a colligative substance property because it depends on the concentration of the solute but not its chemical nature.

Calculating the osmotic pressure formula chemistry is done using, π =iMRT

Step 1: Determining the van ‘t Hoff factor.

Because glucose doesn’t dissociate into ions in the solution, the van ‘t Hoff factor is 1

Step 2: Finding the absolute temperature.

T = Celsius Degrees + 273

T = 37 + 273

T = 310 Kelvin

Step 3: Finding the concentration of glucose.

Π = iMRT

M = Π/iRT

M = 7.65 atm / (1) (0.0820 L.atm/mol.K) (310)

M = 0.301 mol / L

Step 4: Finding the amount of sucrose per liter.

M = mol/Volume

Mol = M·Volume

Mol = 0.301 mol/L x 1 L

Mol = 0.301 mol

Considering the periodic table,

C = 12 g/mol

H = 1 g/mol

O = 16 g/mol

Molar of the glucose = 6(12) + 12(1) + 6(16)

Molar mass of the glucose = 72 + 12 + 96

Molar mass of the glucose = 180 gm/mol

Mass of the glucose = 0.301 mol x 180 gm/1 mol

Mass of the glucose = 54.1 grams


Finally, we should use 54.1 grams per liter of glucose for an intravenous solution to match the 7.65 atm at 37 degrees Celsius osmotic pressure of blood.

What Happens if a Pressure of Higher Magnitude than the Osmotic Pressure applied to the Solution Side?

In this case, the solvent molecules would start moving through the semipermeable membrane from the solution side (the point at the solute concentration is high) to the solvent side (the point at the solute concentration is low). This method is known as reverse osmosis.

Osmotic Pressure Example on Wilting Plants

The Osmotic Pressure example considering the plants can be given in a brief way. Most of the plants use osmotic pressure actually to maintain the shape of their stems and leaves.

If we have kept potted plants, we probably know that our plants can become very wilted very quickly if they are not watered for a long time. But just within minutes of watering, they can perk them right back up!

This happens because the stem and leaves of many plants are fundamentally "inflated" by osmotic pressure – the salts in the cells cause water to be drawn in through osmosis, making the cell plump and firm.

If enough water is not available, the plant will wilt because its cells become "deflated." In scientific terms, they are "hypertonic," which means "the concentration of solute is too high."

Plants can also evidence the power of osmotic pressure as they grow.

We may have seen plants springing up through asphalt, or tree roots growing through concrete or bricks.

Possibly, this, too, is made by osmotic pressure: as plants grow, their cells draw in more water. The slow but inexorable pressure of water moving through the plant cell's membranes can actually push through asphalt!

FAQ (Frequently Asked Questions)

1. Is Osmosis a Reason for Cholera?

Osmosis allows for terrible things to happen. Without osmosis, cholera would not be possible. The bacteria of choleric populate in our intestines and begin to reverse the intestinal cells' ionic orientation. In different terms, it changes the way ions and, subsequently, water transport in our intestines. As a result, cholera performs a perfect coup.

Firstly, when our ions' orientations are switched, the intestinal cells are no longer able to happen water absorption into the body. Now osmosis happens in another direction, and water moves from our intestinal cells into our intestines. This causes cholera's infamously deadly watery diarrhea. Secondly, this compounds the rate at which you become dehydrated. Not only you cannot absorb water, but also you are literally being drained dry. Resultantly cholera can kill you much quicker because it doesn't rely on how much water you consume.

2. Explain about Osmotic in Terms of Water Potential

Water potential is the measure of pressure, ionization, molar concentration, and temperature on one side of a semi-permeable membrane. Comparing the water potential inside a cell to that of outside a cell, the direction of osmosis can be determined.

There is a simple model to represent the relationship between water potential in a cell and the water potential of a cell's extracellular environment.

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Water potential is a quantitative measure of one side pressure of a membrane. By comparing the water potential inside a cell or tissue, with the water potential outside a cell or tissue, or the direction of osmosis can be determined.

The symbol for water potential is psi for Poseidon (ψ), which is called the Greek god of the water.