Osmolarity (or if working in molal rather than molar concentrations, osmolality)

Question

Osmolarity (or if working in molal rather than molar concentrations, osmolality) is a quantity that refers to the concentration of dissolved species in a solution. In your body, normal osmolarity of blood serum is just under 0.3 molar. Because cell membranes are permeable to water, normally the osmolarity of intracellular fluid is approximately equal to that of extracellular fluid (though the intracellular and extracellular composition in terms of specific ions and proteins is quite different). As an aside, measurement of osmolarity of serum is an important clinical test used to check for dehydration or other imbalances of water and electrolytes.

a. In class we briefly discussed osmotic processes, and in the lecture notes a formula was provided for osmotic pressure as a function of solute concentration (Topic 5, Equation (5.30) including text before and after). The solute discussed in the notes was sucrose, but we can consider the same formula to apply for other solutes. Using this formula, calculate the osmotic pressure associated with 0.3 molar concentration of osmolytic particles (ions, proteins, or other dissolved particles) in contact with pure water through a semi-permeable membrane at physiological temperature of 310 K. Express your results in atmospheres of pressure (1 atmosphere of pressure is 1.0x105 Pascal, where 1 Pascal is a Newton/m2).

b. A cell membrane is a bilayer of phospholipid molecules, only a few nanometers thick, and can basically be thought of much like a soap bubble except immersed in aqueous solution. Given the pressure calculated in part a, does it seem reasonable that if cells are placed in pure water, osmotic stress induced by water moving from outside to inside across the semi-permeable cell membrane often causes them to swell and burst in a few minutes? Explain your answer.

Explanation / Answer

a..

We calculate the osmotic pressure, (pi), using the following equation:

pi= MRT

Where:

M is the molar concentration of dissolved species (units of mol/L). = Given, 0.3 mol/L
R is the ideal gas constant (0.08206 L atm mol-1 K-1, or other values depending on the pressure units).
T is the temperature on the Kelvin scale. = 310K

pi = 0.3 * 0.08206 * 310 = 7.63atm

b.. Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.In the above case, this minimum pressure is 7.63 atm. Osmotic pressure of pure water is 0. Water moves from a region of low osmotic pressure to a region of high osmotic pressure, until the osmotic pressure equals on both the sides.

This may never happen in this case, because no solute is being added to pure water; so, osmotic pressure of outside pure water will remain 0; it will continue moving in, until the cell bursts.

Related Questions

Navigate

• Browse (All)
• Browse O
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.