Osmosis and Osmotic Pressure of the Solution: Should you drink seawater if you’re stranded in the middle of the ocean in a life raft? As water from your cells was taken out to dilute the salty ocean water you swallowed, you would feel thirstier. This is due to the osmosis process. We will look at many aspects of the osmosis process in this article.

What is Osmosis?

Consider a pure solvent and a solution separated by a semipermeable membrane that allows solvent molecules to flow but not solute molecules. Only the solvent will diffuse into the solution via the membrane. Semipermeable membranes are those that are permeable to the solvent but not to the solute. Fine holes called capillaries are present in all semipermeable membranes. These allow solvent molecules to flow through but not bigger solute molecules.

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Osmosis is the flow of a solvent from a pure solvent to a solution, or from a dilute solution to a concentrated solution, through a semipermeable membrane.

Fastening a piece of cellophane over a thistle funnel, as shown in Fig., can demonstrate the phenomenon of osmosis. The thistle funnel is filled with a concentrated aqueous sugar solution before being immersed in water. The water starts moving through a semipermeable membrane into the sugar solution. The flow of water into the funnel is visible as the solution rises dramatically in the tube.

What is Osmotic Pressure?

A porous pot with a cupric ferrocyanide membrane is equipped with a rubber stopper and a long glass tube (Fig. a). It contains a sugar solution and is submerged in distilled water. Osmosis causes water to pass through the membrane and into the sugar solution. As a result, the solution level in the long tube rises with time. The hydrostatic pressure formed by the sugar solution column balances out the flow of pure water into the solution after a few days.

Osmotic pressure is the hydrostatic pressure built upon a solution that blocks the osmosis of pure solvent into the solution across a semipermeable barrier. Osmosis can be prevented not only by applying hydrostatic pressure to the solution but also by applying external pressure to the solution. The external pressure can be regulated to prevent the osmosis of water into the solution.

Another definition of osmotic pressure can be found here. The external pressure applied to a solution in order to stop the osmosis of solvent into a solution separated by a semipermeable membrane is known as osmotic pressure. This process is explained in Fig. b. 

Isotonic Solutions

The solutions are considered to be isotonic when they are separated by a semipermeable barrier with no movement of water over it. Isotonic solutions are also iso-osmotic or have the same osmotic pressure if the membrane is entirely semipermeable. The isotonic solutions have equimolar concentrations.

If one of the two solutions separated by a semipermeable membrane has a lower osmotic pressure than the other, it is said to be hypotonic in comparison to the other. It is said to be hypertonic compared to the second solution if it has a higher osmotic pressure than the second solution. Water pours into red blood cells when they are placed in distilled water (hypotonic media), causing them to swell or burst.

When they are placed in a \(5\% \) NaCl solution (hypertonic medium), water leaks out of the cells, causing them to shrink. Blood cells are isotonic to a \(0.16\,{\rm{M}}\) sodium chloride solution \(\left( {0.95{\rm{ }}\% } \right)\), and they do not swell or shrink because no osmosis occurs.

Theories of Osmosis

(1) The Molecular Sieve Theory

According to this notion, the membrane contains many fine pores that operate as a “molecular sieve.” Larger solvent molecules cannot flow through the pores; however, smaller solvent molecules can. Solvent molecules pass through such a membrane from an area with a higher solute concentration to one with a lower concentration. However, some membranes can serve as sieves even when the solute molecules are smaller than the solvent molecules, casting doubt on this notion.

(2) Membrane Solution Theory

Membrane proteins with functional groups like –\(\mathrm{COOH},-\mathrm{OH},-\mathrm{NH}_{2}\), and others use hydrogen bonding or chemical interaction to dissolve water molecules. As a result, the membrane dissolves water from pure water (solvent), generating a so-called “membrane solution.” In order to equalise concentrations, dissolved water flows into the solution through the membrane. Water molecules pass through the membrane in this manner, while solute molecules, which are insoluble in the membrane, do not.

(3) Vapour Pressure Theory

It implies that a semipermeable membrane contains a large number of small pores or capillaries. Water (solvent) or solution does not wet the walls of these capillaries.

As a result, neither the solution nor the water can pass through the capillaries. As a result, each capillary will have the solution at one end and water at the other, with a little space between them. Because a solution’s vapour pressure is lower than that of a pure solvent, vapour will diffuse over the gap from the water to the solution. Water will be transferred into the solution as a result of this. In most circumstances, the vapour theory provides a sufficient explanation of the osmosis mechanism.

(4) Membrane Bombardment Theory

According to this idea, osmosis is induced by an unequal bombardment pressure from solvent molecules on both sides of the semipermeable membrane. On the one hand, we have merely solvent molecules, while on the other, we have solute molecules that take up some of the surface areas. As a result, on the solution side, there are fewer bombardments per unit area of surface than on the solvent side. As a result, on the solution side, the solvent molecules will diffuse more slowly across the membrane than on the solvent side. As a result, the solvent flows through the membrane from the pure solvent to the solution.

Reverse Osmosis

Osmosis of water from water to solution happens when a solution is separated from pure water by a semipermeable membrane. This osmosis can be stopped by applying pressure to the solution equivalent to the osmotic pressure. If a pressure greater than the osmotic pressure is applied, osmosis is forced to work in the opposite direction from regular osmosis, that is, from solution to water.

Reverse osmosis is the osmosis that occurs from a solution to pure water when a pressure larger than the osmotic pressure is applied to the solution.

Laws of Osmotic Pressure

Van’t Hoff demonstrated that for dilute solutions:

(a) The osmotic pressure of a solution at a particular temperature is directly proportional to its concentration based on a review of Pfeffer’s experimental results.

(b) The absolute temperature is directly proportional to the osmotic pressure of a solution of a certain concentration.

Van’t Hoff \((1877)\) formulated the laws of osmotic pressure based on the preceding results, pointing out that they were closely related to the gas laws.

(1) Boyle-Van’t Hoff Law for Solutions

If \(\pi \) is the osmotic pressure and \({\rm{C}}\) its concentration, from \((a)\), we can write \({\rm{\pi }} \propto {\rm{C}}\) if the temperature is constant. If the concentration of the solute is expressed in moles per litre and \({\rm{V}}\) is the volume of the solution that contains \(1\) mol of solute.

\({\rm{C = }}\frac{{\rm{1}}}{{\rm{V}}}\)

Thus,

\({\rm{\pi }} \propto \frac{{\rm{1}}}{{\rm{V}}}\) at constant temperature

(2) Charles-Van’t Hoff Law for Solutions

If \({\rm{T}}\) is the absolute temperature, from the statement (b), we can writ

\({\rm{\pi }} \propto {\rm{T}}\) if the temperature is constant

(3) Van’t Hoff Equation for Solutions

As shown above, the osmotic pressure \(({\rm{\pi }})\) of a dilute solution is inversely proportional to the volume \({\rm{(V)}}\) containing \(1\) mole of the solute. It is directly proportional to the absolute temperature \({\rm{(T)}}\). This is

\({\rm{\pi }} \propto \frac{{\rm{1}}}{{\rm{V}}}\) (1)

\({\rm{\pi }} \propto {\rm{T}}\) (2)

Combining (1) and (2) Van’t Hoff gave the general relationship,

\({\rm{\pi V = nRT}}\)

Where \({\rm{n = }}\) Number of moles of solute

\({\rm{R = }}\) Gas constant

Determination of Molecular Weight from Osmotic Pressure

Knowing the osmotic pressure of a given solution, the molecular weight of the solute can be calculated as follows from Van’t Hoff equation.

\({\rm{\pi V = nRT}}\)

\({\rm{ = }}\frac{{\rm{w}}}{{\rm{M}}}{\rm{RT}}\)

\({\rm{M = }}\frac{{{\rm{wRT}}}}{{{\rm{\pi V}}}}\)

\({\rm{M = }}\) molecular mass of the solute

\({\rm{w = }}\) amount of solute in grams

\({\rm{R = }}\) \(0.0821\) litre-atmosphere

\({\rm{T = }}\) \(\left( {{\rm{t}}\,^\circ {\rm{C}} + 273} \right){\rm{K}}\)

\({\rm{\pi = }}\) osmotic pressure in atmospheres

\({\rm{V = }}\) volume of solution in litres

Summary

1. Osmosis is the flow of a solvent from a pure solvent to a solution, or from a dilute solution to a concentrated solution, through a semipermeable membrane.
2. The external pressure applied to a solution in order to stop the osmosis of solvent into a solution separated by a semipermeable membrane is known as osmotic pressure.
3. The solutions are considered to be isotonic when they are separated by a semipermeable barrier with no movement of water over it.
4. Knowing the osmotic pressure of a given solution, the molecular weight of the solute can be calculated.

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FAQs on Osmosis and Osmotic Pressure of the Solution

Q.1. What is osmosis?
Ans: The process of osmosis can be defined as the flow of a solvent from a pure solvent to a solution, or from a dilute solution to a concentrated solution, through a semipermeable membrane.

Q.2. What is osmotic pressure?
Ans: Osmotic pressure can be defined as external pressure applied to a solution in order to stop the osmosis of solvent into a solution separated by a semipermeable membrane.

Q.3.  What are isotopic solutions?
Ans: The solutions are considered to be isotonic when they are separated by a semipermeable barrier with no movement of water over it. Isotonic solutions are also iso-osmotic or have the same osmotic pressure if the membrane is entirely semipermeable. The isotonic solutions have equimolar concentrations.

Q.4. What is the effect of osmosis on red blood cells?
Ans: It is interesting to see the effect of osmosis on red blood cells. Water pours into red blood cells when they are placed in distilled water (hypotonic media), causing them to swell or burst. When they are placed in a \(5\% \) NaCl solution (hypertonic medium), however, water leaks out of the cells, causing them to shrink. Blood cells are isotonic to a \(0.16\,{\rm{M}}\) sodium chloride solution \(\left( {0.95{\rm{ }}\% } \right)\), and they do not swell or shrink because no osmosis occurs.

Q.5. How can the molecular weight of solute be measured using osmotic pressure?
Ans: By knowing the osmotic pressure of a given solution, the molecular weight of the solute can be calculated as follows from Van’t Hoff equation.

\({\rm{\pi V = nRT}}\)

\({\rm{ = }}\frac{{\rm{w}}}{{\rm{M}}}{\rm{RT}}\)

\({\rm{M = }}\frac{{{\rm{wRT}}}}{{{\rm{\pi V}}}}\)

\({\rm{M = }}\) molecular mass of the solute

\({\rm{w = }}\) amount of solute in grams

\({\rm{R = }}\) \(0.0821\) litre-atmosphere

\({\rm{T = }}\) \(\left(t^{\circ} \mathrm{C}+273\right) \mathrm{K}\)

\({\rm{\pi = }}\) osmotic pressure in atmospheres

\({\rm{V = }}\) volume of solution in litres

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