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

  • #1

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.
 

Answers and Replies

  • #2
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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..
 
  • #3
rude man
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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.

Also, careful about units as again admonished.

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:

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