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.

Osmotic Pressure Equation

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

π=iCRT.

Where,

• ‘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.

What Exactly is Osmosis?

The minimal pressure required for a solution to stop the passage of solvent molecules through a semipermeable membrane is known as the osmotic pressure. The passage of solvent molecules through a membrane from a low-concentration segment to a high-concentration segment is referred to as the osmosis process. The two sides of the semipermeable membrane eventually achieve equilibrium due to this process.

The semipermeable membrane is a type of membrane that allows solvent molecules to pass through while preventing solute particles from passing through. This osmosis process will come to a halt if we apply enough pressure to the solution side of the semipermeable membrane.

The osmotic pressure is defined as the minimal amount of pressure required to stop the osmosis process. This method will be repeated until the concentrations of the two solutions are equal. It could also happen if the increased pressure prevents any additional water from passing through the membrane.

Understanding the Osmotic Pressure

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

(Image will be uploaded soon)

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?

Explanation:

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

Answer:

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.

Examples and Instances

• Plants rely on osmotic pressure to keep their upright structure, also referred to as turgidity. When the plant receives enough water, its cells absorb the water and expand thus becoming turgid and they hold the shape of the various parts firmly. When the cells expand by absorbing water, it ultimately increases the pressure on their cell walls.

• When a plant receives insufficient water, its cells become hypertonic and they shrink due to water loss. They wilt and lose their solid, erect posture, a situation known as flaccidity.

• Desalination and purification of saltwater, which requires reverse osmosis, is another notable application of osmotic pressure.

• Our fingers become wrinkly when we sit in the bathtub or submerge them in water for an extended period of time. This is due to osmosis as well. Our fingers' skin absorbs water and expands or bloats, resulting in pruned or wrinkled fingers.

• Slugs and snails are killed by sprinkling salt on them. It involves the osmosis process, which kills them. They end up shedding water as the liquid inside them tries to dilute the salt concentration and preserve the mucus layer. Slugs and snails will dry up and die if they are exposed to too much salt

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 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!