# What is the osmotic pressure? (intro life-science physics)

• duckological
In summary, the conversation discusses the concept of monovalent salt and how to find the concentration of individual ions within a solution. There is also a mention of the pressure difference between inside and outside the cell, which is represented by the symbol ##\pi## in an equation. It is advised to be careful about the ionic dissociation of the salt and to pay attention to units. The term "monovalent" refers to the number of electrons in the outermost band of the molecule.
duckological
Homework Statement
The monovalent salt concentration (the predominant solute in the blood cell) for a sample of red blood cells is 0.21 moles/liter. If one of these red blood cells were placed in pure water (at around room temperature, 300 K), and the cell comes to hydrostatic equilibrium with the water, what is the osmotic pressure of the cell (assuming it doesn't burst)? Again, be careful, salts dissociate when dissolved, and take care with the concentration (moles/liter vs moles/m3).
Relevant Equations
π=CRT
where:
π is osmotic pressure
C is concentration
R is the constant 8.314 J/Mol*K
T is the temperature at 300 K
I haven't learned what a monovalent salt is, but through some research, I'm assuming it's when a solute like NaCl dissociates into one of each molecule, so C_solute is equal to C_Na + C_Cl.

With that assumption, how do I find C_Na and C_Cl?

If I read the question correctly, the sample of red blood cells is the same as salt (0.21 moles/liter), but then it says the red blood cells were placed in pure water, which is where I am lost.

Again, I'm just confused on what C is.

The pressure inside the cell has to be higher than in the pure water surrounding the cell to prevent the outside water from coming in through the cell membrane to try to dilute the salt solution inside the cell. C is the concentration of the molar NaCl inside the cell liquid. The pressure difference between inside and outside the cell is the ##\pi## in your equation..

Considering the admonition of being careful about ionic dissociation of the salt in water, you should assume that your given molar density doubles from the given (un-ionized) concentration. Which i think is as you suspected. So C doubles.

BTW 'monovalent' means 1 electron per molecule (or atom) in the outermost (valence) band, e.g. NaCl.
edit: SCRATCH THAT LAST SENTENCE: If it had been a bivalent salt for example you would have gotten 4 times the unionized concentration. Etc.

Last edited:

## 1. What is osmotic pressure?

Osmotic pressure is a physical phenomenon that describes the pressure required to prevent the movement of water across a semipermeable membrane. It is a result of the concentration gradient of dissolved particles on either side of the membrane.

## 2. How does osmotic pressure affect living organisms?

Osmotic pressure is crucial for maintaining the balance of fluids and ions within a living organism. It helps regulate the movement of water and nutrients across cell membranes, which is essential for proper cell function.

## 3. What is the relationship between osmotic pressure and concentration?

The osmotic pressure is directly proportional to the concentration of solute particles in a solution. This means that the higher the concentration of solute, the higher the osmotic pressure and vice versa.

## 4. Can osmotic pressure be measured?

Yes, osmotic pressure can be measured using a device called an osmometer. This instrument measures the amount of pressure needed to balance the movement of water across a semipermeable membrane.

## 5. How is osmotic pressure related to osmosis?

Osmotic pressure is the driving force behind osmosis, which is the movement of water across a semipermeable membrane. The higher the osmotic pressure, the greater the rate of osmosis.

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